When you try to push and you get this message that’s mean that someone else has changed after your last fetch.
Well in this case you have to incorporate their changes before you can add yours typically, you’ll want to rebase rather than merge and you’ll also want to test the joined version.
$ git fetch
$ git diff master…origin/master
that’ll show changes of upstream
$ git fetch origin
$ git rebase origin/master
$ git push prigin master