How do I reset the time delay on my Veritas R8 house alarm?

When my house alarm was fitted, the beep that sounds when the alarm is set, was set to last for 30 seconds. This is an overly long time and the neighbours have complained, hence I want to reduce the time it beeps for. I can't work out how to do it from the manual and the original fitter doesn't seem to want to come back and do it for me. Can anyone help please?