Contribute to an existing branch on GitHub

# assumption: a project called `repo` already exists on the machine, and a new branch has been pushed to since the last time changes were made locally

# change into the `repo` directory
cd repo

# update all remote tracking branches, and the currently checked out branch
git pull

# change into the existing branch called `feature-a`
git checkout feature-a

# make changes, for example, edit `` using the text editor

# stage the changed file
git add

# take a snapshot of the staging area
git commit -m "edit file1"

# push changes to github
git push