# if you updated your files on github.com so may be thats its asking first pull
# request then your push the request.
# so Very simple first:
# not use -f 
git pull origin master
# then
git push origin master

git push -f origin master !!This is never a recommended use of git. This will overwrite changes on the remote. Only do this if you know 100% that your local changes should be pushed to the remote master.