How to you change the timing belt of a 1987 Nissan Sentra?
The question you ask is too involved for a proper answer. You may want to go to a car parts store, and get a repair manual for your car. They cost about $16.00 Or, go to a Public Library.
Can you home-diagnose and repair why the light is on which only says 'Sensor' on a 1987 NIssan Sentra?
If you are talking about ignition timing then the answer is easy. You don't. Suzuki's PEI (Pointless Electronic Ignition) takes care of timing the spark. If you are talking about valve timing-resubmit and I will take a stab at explaining it. There is no way to remap the electronic ignition system to change the spark timing.