Changing source branch for PR on Github

AFAIK, you cannot change the source branch after creating a Pull Request. You have to create a new one instead.

For future reference, the established best practice is to create a new branch before making any commits. You should not commit directly to master, especially when contributing to team projects.

Side note:

you can simply Add more commits by pushing to the master branch on username/repo

More correctly, any changes to the target branch of a PR are automatically included in the PR.

Since you can't change the source branch in github (you can only change the target branch), you need to update the source-branch "inplace".

This answer contains three ways of doing this.

In Github you see:

githubuser wants to merge N commit into master from "source-branch"

Since you will change the history of "source-branch", be sure nobody else is using this branch!

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Rewrite history of source-branch

If you are the only one developing on this branch, then you can use git rebase -i and later git push --force-with-lease.

This way you can rewrite the history of this branch.

Docs about rewriting the git history: https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History

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Rename temporary branch to source-branch

If you prefer to start from scratch.

git checkout master

# create a temporary branch
git checkout -b tmp-branch

... now modify the tmp-branch the way you want to. Commit, but don't push.

# rename tmp-branch to "source-branch"
git branch -f -m source-branch

# Be sure nobody is using "source-branch", since the history gets rewritten.
git push --force-with-lease

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git rebase --onto

Imagine your branch "source-branch" was accidentely based on "release" and not on "master". But the github target branch is "master". This way you can change your changes to be on top of master:

git checkout source-pr
git rebase --onto master release