If you have merged a branch in Git, but you haven't pushed the changes to the remote repository yet, you can undo the merge by using the following command:
git reset --hard HEAD^
This will reset your local branch to the previous commit, effectively undoing the merge. The --hard option will discard any changes you have made in your working directory, so make sure you have committed any changes you want to keep before running this command.
If you want to keep the changes in your working directory, you can use git reset --soft HEAD^ instead. This will undo the merge, but leave your changes staged in the working directory.
After undoing the merge, you can make any additional changes you need, commit them, and then push your changes to the remote repository. However, if other people have already pulled the merged changes, you will need to communicate with them about the changes you are making and coordinate with them to avoid conflicts.