Create user token in user settings

Set this token as a secret USER_TOKEN in settings of a target repository: https://github.com/<owner>/<repo>/settings/secrets

create file .github/workflows/pr-updater.yml with the following content:

name: PR update

on:
    push:
        branches: 
            - master

jobs:
    autoupdate:
        runs-on: ubuntu-latest
        steps:
        - uses: actions/checkout@v1
        - name: update all prs
            uses: maxkomarychev/pr-updater-action@v1.0.0
            with:
                token: ${{ secrets.USER_TOKEN }}

Now every time code is pushed to branches specified in the workflow all ohter pull requests targeting these branches will be automatically updated.