How do you adjust idle speed on 1998 Chevy?

You can't. It is not adjustable. It is computer controlled.

You need to have the engine scanned with an engine scanner. If I was you I would clean the throttle plate real good with an old tooth brush and carb. cleaner until it is shinny and clean. That may just fix your idling problem.

UNLESS ....

If it has been modified and replaced with a carb you can adjust your idle throttle with the screw located behind the throttle cable, left to increase throttle, right to decrease.