diff --git a/econml/_ensemble/_ensemble.py b/econml/_ensemble/_ensemble.py
index e08ee8983..8217ad074 100644
--- a/econml/_ensemble/_ensemble.py
+++ b/econml/_ensemble/_ensemble.py
@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code is a fork from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/ensemble/_base.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/ensemble/_base.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/_tree_exporter.py b/econml/_tree_exporter.py
index b7f536e80..e1d14f1d9 100644
--- a/econml/_tree_exporter.py
+++ b/econml/_tree_exporter.py
@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains some snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_export.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_export.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/grf/_base_grf.py b/econml/grf/_base_grf.py
index e6841decf..df48d0bc4 100644
--- a/econml/grf/_base_grf.py
+++ b/econml/grf/_base_grf.py
@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/ensemble/_forest.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/ensemble/_forest.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/grf/_base_grftree.py b/econml/grf/_base_grftree.py
index 4454420d1..d65381bd9 100644
--- a/econml/grf/_base_grftree.py
+++ b/econml/grf/_base_grftree.py
@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_classes.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_classes.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/grf/_criterion.pyx b/econml/grf/_criterion.pyx
index 74504e0ca..59a448a68 100644
--- a/econml/grf/_criterion.pyx
+++ b/econml/grf/_criterion.pyx
@@ -6,7 +6,7 @@
 # Licensed under the MIT License.
 #
 # This code contains some snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_criterion.pyx
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_criterion.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/policy/_forest/_forest.py b/econml/policy/_forest/_forest.py
index e8d4bad7f..28cd6b194 100644
--- a/econml/policy/_forest/_forest.py
+++ b/econml/policy/_forest/_forest.py
@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/ensemble/_forest.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/ensemble/_forest.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/policy/_forest/_tree.py b/econml/policy/_forest/_tree.py
index 74531b3b6..0c6be50b8 100644
--- a/econml/policy/_forest/_tree.py
+++ b/econml/policy/_forest/_tree.py
@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_classes.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_classes.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/tree/_criterion.pyx b/econml/tree/_criterion.pyx
index c50c2fef3..49fe79bab 100644
--- a/econml/tree/_criterion.pyx
+++ b/econml/tree/_criterion.pyx
@@ -5,7 +5,7 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 #
-# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_criterion.pyx
+# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_criterion.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/tree/_splitter.pyx b/econml/tree/_splitter.pyx
index b07e05987..8b7ae406d 100644
--- a/econml/tree/_splitter.pyx
+++ b/econml/tree/_splitter.pyx
@@ -5,7 +5,7 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 #
-# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_splitter.pyx
+# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_splitter.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/tree/_tree.pyx b/econml/tree/_tree.pyx
index 2b1c5c755..07daf4cff 100644
--- a/econml/tree/_tree.pyx
+++ b/econml/tree/_tree.pyx
@@ -5,7 +5,7 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 #
-# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_tree.pyx
+# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_tree.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #
diff --git a/econml/tree/_utils.pyx b/econml/tree/_utils.pyx
index e9fb95e48..d9e9968ac 100644
--- a/econml/tree/_utils.pyx
+++ b/econml/tree/_utils.pyx
@@ -5,7 +5,7 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 #
-# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_utils.pyx
+# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_utils.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #