When deleting, you can specify a member by last name, first name(s), or both (or even URL!), but they must exactly match the member list; you may wish to copy/paste from the member list. And if you delete using only one name, all students with that name will be deleted!
In general, it should be rare that students get deleted; instead, they should probably be moved to alumni.
The Update button will regenerate the member's list file. This is useful if you make a formating change in the addmember.cgi script and wish to update the member's list to reflect this change.