Git encourages branching but sometimes you need to do a clean up and delete merged or orphaned branches from your local and remote repositories. Here is the procedure from the command line.
To delete a branch locally:
git branch -d branchname
You may get an error like this one if the branch has not been merged yet:
error: The branch 'branchname' is not fully merged. If you are sure you want to delete it, run 'git branch -D branchname'
This is a good reminder in case you are deleting something you should not be deleting.
To delete a branch remotely (e.g. on a GitHub repository):
git push origin --delete branchname