Your question doesn't make sense to me. If you want a higher ASA (or ISO, same thing), setting, go into the menu and raise the setting. All other things being equal, you'll then get a higher shutter speed.
How do I change the default shutter speed that will force the Nikon D50 to select a higher ASA ratings?
Check the index of your manual for AUTO ISO. I know the D50 has this feature.
How do I change the default shutter speed that will force the Nikon D50 to select a higher ASA ratings?
Try using Shutter priority (S mode). This will put you in control of the shutter speed (set manually) and the rest will be set automatically (including the ISO)
