This is achieve by
git mergewhile on the master branch.
To be sure to not mess up anything, I prefer to clone my repository first and do the merge there.
So best is if you commit all changes in master and dev branch. Do a couple of pushs and pulls so everything is in sync and then clone your repository to a new location.
You can directly clone the branch you want to merge then proceed to 2. otherwise do a
git checkout branchname
git checkout master
git merge branchname
You have to resolve any conflicts in any files by removing the spiky brackets in the file and of course take a close look what caused the conflict and fix it.
Then add the file via
git addand commit it via
Do this for every file.