How do I replace the check engine light in a.

Obviously it's working so you don't replace it. The Engine Computer detected a problem, set a diagnostic fault code, and turned the light on to tell you. The place to start is by having the code(s) read. That will indicate the circuit or system that needs further diagnosis.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method