You will need to goto the COMPOSITION SETTINGS for the comp you wish to change.
Either right click the comp and select the option from the footage panel
Or when you have the comp open and selected press ctrl K or mac equivelent (or the menu composition - composition setting)
Near the bottom of the window that open will be a box often overlooked that says start frame, you can change it here.
Hope this is accurate I'm doing it from memory.
Regards
Chris Forrester
demoreel
www.chrisforrester.tv