Git merge in wrong direction

Reverting a merge on the branch is as simple as determining which commit was the previous one.

First, checkout on the branch you want to modify. It's a git-golden-rule: you can only modify the branch you're on.

git checkout master

Then, figure out the previous-master commit it, and force master back into its previous state

git reset --hard <previous-commit-id>

Finally, do your merge right

git checkout devel
git merge master

---

What you could also do, if you want to be lazy, is invert branch states.

You still need to find master's previous commit-id.

Then:

git checkout devel
git reset --hard master
git checkout master
git reset --hard <previous-commit-id>