The simplest way to get some commits from one branch to another is cherry-picking.
Assuming that your fix in master has the commit hash HASH and you want to take that hotfix into your devel branch, do a git checkout devel followed by a git cherry-pick HASH. That’s it.
If you want to take all changes from master into devel, you can achieve that with
git checkout devel
git rebase master
If you have the opposite scenario (you make a hotfix during development in a devel branch and want to take that fix into master before devel gets fully merged into master), the workflow is quite similar. Assuming that the hotfix has the hash HOTFIX_HASH, do this:
git checkout master
git cherry-pick HOTFIX_HASH
Now, the commit is present in master and devel. To get around this, type
git checkout devel
git rebase master
and the commit will disappear from devel since it’s already present in master.