If the broken code is only on the second branch, you can safely delete that branch without affecting your main (or master) branch.

Using the GitHub website:

Open your repository.
Go to Branches.
Find the branch you want to remove.
Click the Delete (trash can) icon next to that branch.

Using Git:

git checkout main
git branch -d branch-name

If Git refuses because the branch isn't merged and you're sure you want to remove it:

git branch -D branch-name

To delete the remote branch as well:

git push origin --delete branch-name

Deleting a branch does not delete or modify your main/master branch. Just make sure you don't need any commits that exist only on the branch you're deleting