There's a flowchart in my System 3 manual, but I don't have a scanner handy. Before I paraphrase it (or somebody else gets there first), is it the same alarm and are you sure there isn't already a PIN (you need to know the existing PIN to change it).
Occurs to me that if you have a Datatool alarm it's good practice to set up a PIN. Otherwise anybody with the remote and 5 minutes could set one they know and override the alarm at their convenience.