Currently, Set-DRSClusterGroup does not appear to allow setting the name/label of a DRS VM/Host Group. Something like the following should work to modify the group name:
$currentGroupObj = Get-DRSClusterGroup -Name "MyCurrentGroupName"
$updatedGroupObj = $currentGroupObj | Set-DRSClusterGroup -Name "MyNewGroupName" -Confirm:$false
I would consider this a higher need since I am not aware of ANY facility to rename a DRS VM/Host Group name.