After installation simply comment /rebase to trigger the action:

To configure the action simply add the following lines to your .github/workflows/rebase.yml workflow file:

name: Automatic Rebase
on:
  issue_comment:
    types: [created]
jobs:
  rebase:
    name: Rebase
    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase')
    runs-on: ubuntu-latest
    steps:
      - name: Checkout the latest code
        uses: actions/checkout@v2
        with:
          token: ${{ secrets.GITHUB_TOKEN }}
          fetch-depth: 0 # otherwise, you will fail to push refs to dest repo
      - name: Automatic Rebase
        uses: cirrus-actions/[email protected]
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

NOTE: To ensure GitHub Actions is automatically re-run after a successful rebase action use a Personal Access Token for actions/checkout@v2 and cirrus-actions/[email protected]. See the following discussion for more details.

Example

... 
    - name: Checkout the latest code
      uses: actions/checkout@v2
      with:
        token: ${{ secrets.PAT_TOKEN }}
        fetch-depth: 0 # otherwise, you will fail to push refs to dest repo
    - name: Automatic Rebase
      uses: cirrus-actions/[email protected]
      env:
        GITHUB_TOKEN: ${{ secrets.PAT_TOKEN }}

You can also optionally specify the PR number of the branch to rebase, if the action you're running doesn't directly refer to a specific pull request:

    - name: Automatic Rebase
      uses: cirrus-actions/[email protected]
      env:
        GITHUB_TOKEN: ${{ secrets.PAT_TOKEN }}
        PR_NUMBER: 1245

It's possible to use author_association field of a comment to restrict who can call the action and skip the rebase for others. Simply add the following expression to the if statement in your workflow file: github.event.comment.author_association == 'MEMBER'. See documentation for a list of all available values of author_association.

GitHub can also optionally dismiss an existing review automatically after rebase, so you'll need to re-approve again which will trigger the test workflow. Set it up in your repository Settings > Branches > Branch protection rules > Require pull request reviews before merging > Dismiss stale pull request approvals when new commits are pushed.