You can delete a snapshot from the snapshots shot view by clicking on the icon following the snapshot you wish to remove.
Or, from the volumes view, click on the target volume from which the snapshot was made, then click on the "Snapshots" button to view all of the snapshots associated with that volume. You can delete a snapshot by clicking on the after the one you wish to purge.
If you wish to delete a range of snapshots, select "Clear Snapshots". You will be prompted to enter a date from which to begin. All snapshots created before that date will be purged.
Note: Once a snapshot is purged, it is gone forever. Make certain the snapshot you are about to delete is the correct one. Make sure you are entering the correct date.