LinuxQuestions.org - A git branch meant to be alternative fail to be swapped

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A git branch meant to be alternative fail to be swapped

I made a branch meant to be experimental and alternative to master so did

Code:

git branch M

after it's been worked, it turns out to be better, so did swap:

Code:

git branch  -m master old 

git branch  -m M master

but when ready, did push, suddenly

fatal: The upstream branch of your current branch does not match
the name of your current branch. To push to the upstream branch
on the remote, use

git push origin HEAD:M
...

It didn't get synchronized into remote git.
How's the correct one, will anyone point out the important missing/subtle step ?

you forgot to tell us which command failed "did push ... fatal" is not enough.
From the other hand the usual way of working is to merge content and do not swap branch names. Especially altering master is not a good idea at all.