First of all, Hemscott is no modem manufacturer.
Secondly, open Hyperterminal, hier you need to establish a pseudo connection and hang up again.
than just enter AT followed by the switch to change and = and the new value.
It will be stored in the modems memory.
Most modems work with a Rockwell chip, i need to search to find the old commands for it. Obviously it is not a new modem....