Using git reset to Undo a Merge:

To undo last merge you only need to checkout last commit that was done before merging any other  branch in to current branch using git reset --hard commitSHA

git reset --hard commitSHA

If you don't know SHA of last commit you also do that using the following command

git reset --hard HEAD~1

How to Undo a Pushed Merge:

You can also undo a pushed merge that is pushed on your remote repo using the following command.

git revert -m 1 commitSHA

git revert is a better solution in case if you have already pushed to a remote repo.

How to find hash id of your commit

Use the git log command to find the hash (SHA) id of your last commit