update_submodules.yml

# This workflow updates some or all submodules to their latest remote commit on the target branch of this repo

name: Update submodules to latest
on:
 schedule:
    - cron: '0 1 */1 * *' # every day at 01:00
 workflow_dispatch: # on button click

jobs:
 Update_submodules:
    runs-on: ubuntu-latest
    name: Update submodules to latest
    steps:
     - uses: actions/checkout@v2
       with:
        ref: stable                     #target branch
        #submodules: 'recursive'     # possibly needed in your situation  
     - name: Update
       run: |
         set -x
         
         git config --global user.name 'Actions Bot'
         git config --global user.email '<>'
         
         #update all submodules
         #git submodule update --recursive --remote --init
         
         #update only individual submodules
         git submodule update --remote --init Tools/sitl_gazebo
         git add Tools/sitl_gazebo
                  
         
         if [[ `git status --porcelain --untracked-files=no` ]]; then
           # Changes to commit
           git commit -m "Update submodules to latest"
           #push changes
           git push
         else
           echo "no changes to commit"
         fi