Create branch from another’s Pull Request

git fetch upstream pull/354/head:pr354
git checkout -b 10.0-branch-name pr354

More information: https://help.github.com/articles/checking-out-pull-requests-locally/

Push updates to another’s Pull Request

If you have access to edit PR files via github UI, you can push such updates from console

GITHUB_USERNAME=yelizariev  # set username where PR is made from
REPO=pos-addons # set repo name
BRANCH=10.0-fix-something # set source branch name

git remote add ${GITHUB_USERNAME} git@github.com:${GITHUB_USERNAME}/${REPO}.git
git fetch ${GITHUB_USERNAME} ${BRANCH}
git checkout ${GITHUB_USERNAME}/${BRANCH}
# make updates
# ...
# make commit
git commit ...

# push update to another's branch
git push ${GITHUB_USERNAME} HEAD:${BRANCH}