You will be removing the branch directly on GitHub. If you have not merged it back into another branch, the changes in the branch you delete will be lost.
Hold Shift and right-click on your source code directory. Choose TortoiseGit, Browse Reference.
On the left, expand remotes and select origin.
Right-click on the branch you want to delete and select Delete remote branch.
Accept the warnings if you know what you're doing.
