No need to shut down the virtual machine, but the virtual machine will be freezed while creating a snapshot.
Rollback to previous snapshot
same as rollback to previous snapshot,
right-click > export
NOTE : In Hyper-V 2008 R2, we need to shutdown the virtual machine before export.
However in 2012 R2, no need to shutdown the virtual machine.
New > Virtual Machine
and we have the choice to choose the existing HDD
do not forget to set the RAM
right click at snapshot > Delete checkpoint
right click at snapshot > Delete checkpoint subtree
and wait for hyper-v to merge snapshot.