how do adjust and/or replace tappets on an '83 honda cb650?

Answer 1

Honda 1983 CB650SC has hydrolic valve adjusters. There is no manual adjustment.
If the engine is making a noise like the valves need adjusting, then one or more of the hydrolic adjusters will need replacing.
It's not a simple job.
Take it to a mechanic or buy a shop manual.
http://www.motocom.com/motorcycles/