All the git-rebase-fu I have ever needed while working with pull requests, I have found in this well written guide: https://github.com/susam/gitpr

Quoting from this document below.

  # Rebase topic branch on the main development branch (optional).
  git checkout TOPIC-BRANCH
  git rebase master

  # Edit commits, e.g., last 3 commits in topic branch (optional).
  git checkout TOPIC-BRANCH
  git rebase -i HEAD~3

  # Force push rebased/edited commits to the pull request (optional).
  git push -f origin TOPIC-BRANCH