The following is a guide to rename your master branch. It will work just as easily to rename your experimental branch.
Here's how I did the renaming.
First, in your working tree, locally rename master to something
else.
git branch -m master old-dev
Renaming a branch does work while you are on the branch, so
there's no need to checkout something else.
Then, locally rename the maintenance branch (2.63-branch) to
master:
git branch -m 2.63-branch master
Now, time to mess with the remote. Just in case you screw up, you
might want to make sure you have a current backup. First, delete the
remote's master:
git push origin :master
And now, give the remote your new master:
git push origin master:refs/heads/master
Update: When creating a new branch, the refs/heads/ prefix is
needed on the remote side. If the branch already exists (as master did
above) only the branch name is required on the remote side.
... and your now-renamed old master:
git push origin old-dev:refs/heads/old-dev
Finally, delete the old name of your maintenance branch to prevent
confusion:
git push origin :2.63-branch
Clients will now get the 'new' master branch when they pull.