1. Find the commit hash you want to edit

git log 

2. Start interactive rebase, adjust the number if needed git rebase -i HEAD~1

3. In the editor, change 'pick' to 'edit' for the commit you want to modify

4. Save and close the editor

5. Add the new file

git add path/to/newfile 

6. Amend the commit to include the new file git commit --amend

Save and close the commit message editor

7. Continue the rebase git rebase --continue

8. Resolve any conflicts if necessary Force push to the remote repository if needed git push --force