Hi !
It is not possible to rename a group.
The best way to change a group name is to create an other group with the desired name and to move all the clients of the original group to the new one.
Remove the old group from its related pool and Affect the new group to this pool.
You can , if you are very confortable with NetWorker ressource files, stop NetWorker process ( services if you run Windows ) edit the nsr.res file and change every occurence of the group name you want to change by the new name.
Restart NetWorker process and it should work !!!
Cheers,
Suwaker