First, let’s clarify that your title use of the word “Revert” does not necessarily mean the same thing as the Git command revert, which creates a new commit that undoes the changes in a previous commit. I believe your goal is to “Undo” the most recent PR to master, and there are 2 ways to do that, one of which is using the revert Git command.
Option #1: Reset master
This option will remove PR 12943 from master and make it look like it never happened. Since you are resetting and rewriting master, you cannot use your PR functionality to do this, and instead must force push. If you decide to force push master, which is a shared branch, you will need to effectively communicate with all developers of your repo, to explain that you are doing this. You may also need to provide guidance to developers how to rebase their branches in case they have in progress branches already started from the latest master.
Command Line:
git switch master
git reset --hard <commit-id-of-Merged-PR-12923>
# Update permissions on master
git push --force-with-lease # if this fails, fetch and inspect `master` again
Or Visual Studio:
To mimic the above functionality in Visual Studio, with master checked out right click on the commit “Merged PR 12923”, and select the option to do a hard reset. Then push. You may need to configure VS to enable force pushing.
Option #2: Revert PR 12943 from master
This option will create a revert commit which undoes all the changes brought in with PR 12943. This new commit can then be PR’d into master as usual. The history will remain intact. Although Azure DevOps (which it looks like you’re using) offers the ability to revert a PR using the web UI, I find it a cleaner to do it from the command line:
git fetch
# create a new branch starting from master
git switch -c revert-PR-12943 origin/master --no-track
git revert HEAD -m 1 # this will revert the merge commit at your current HEAD
# consider editing the commit message with why you are doing this
git push --set-upstream origin revert-PR-12943
# Now PR revert-PR-12943 into master