Well it's a bit late, but for someone who might have the same issue, like me today, if you are using VirtualStudio Code

1. Go to source control page (on the left side of the screen)
2. Next to the refresh icon, you have 3 dots, click on that icon with the tree dots
3. You'll find every option of GitHub
4. The go to commit, and find the action named "undo last commit"