this probably happens because you've made changes to the detached head.
try forcing the checkout, which will discard the changes, as you mentioned you don't care about those.
git checkout -f MyBranch
from the git help checkout man page:
-f, --force When switching branches, proceed even if the index or the working tree differs from HEAD. This is used to throw away local changes.
When checking out paths from the index, do not fail upon unmerged entries; instead, unmerged entries are ignored.
Switched to branch 'master'
So, you are on master branch now.
Your branch is behind 'origin/master' by 6 commits,
This means that the remote master branch, has updates, and that you may want to update your local branch to match the remote. You have already fetched the origin/master branch, thus git knows that your local branch is behind the remote branch.
and can be fast-forwarded.
This means that the remote branch has linear changes made to it, and that there will be no conflicts in merging/updating your local branch.
To update the local branch, just merge the origin/master branch to it.
git merge origin/master
error: The following untracked working tree files would be overwritten by merge:
this means that you have some files locally, that are not tracked (have never been git add'ed and commited) that the new changes (the merge) is going to add to the git list of files to watch and track. So the new files, that are going to be added, clutter with the local untracked files on the repo.
To resolve that, either remove those local files, or rename them, or move them to another location and try to merge the remote branch again.
