how to undo a git merge?

Hello,

I'm a git lover with a question.

I have two git branches that stem from the same exact master branch.
Many commits happened independently on these two branches overtime.
Then, I inadvertently did a merge from one branch into another. The branch
where I merged into now has mixed commits, and also a commit for the merge
itself. I then had more commits added to the branch that I merged into.

Now I would like to revert the merge... what is the best way of doing that?
I would like very much to use something like git-revert <merge_commit_id>
But that does not seem to work.

Any help/pointers of an example illustrating solution to this case
would be greatly
appreciated!

Regards,

-- ff@member.org