What is the procedure for changing the ignition switch in a 2000 Chevy S10 truck?

Remove the dash trim around the gauge cluster and radio. It helps to put the shifter in 1st to get it out. Remove the upper and lower steering column covers. Disconnect the air bag(unhook the battery first). You'll have to remove the steering wheel to gain access to one of the bolts that holds the tumbler housing to the column. I think there are 3 in total, they are torx heads. Remove those bolts and the housing will come off.

If I misunderstood the question and all you want to replace is the plastic switch that is turned by the tumbler when the key is turned. You can get to that by just removing the plastic around the column. Below the key you will see the big wiring harness.(disconnect the battery) This piece can be pulled out of the bottom of the tumbler housing with a screw driver. Once it's out you will see what the tumbler actually is turning when you turn the key.