If it has coil packs, you can't.
It is electronic controlled and requires an OBDII engine scanner to set the timing if it has a distributor. If it don't have 1 then there is NO adjustment for the timing. You did not give the engine size.
Its at the rear of the engine and a pain to work on where its wedged in.
Assuming that it has a 4.3L engine, it would be the distributor.
Gonna need to know which engine you have in there.
What year the vehicle is would be helpful and what engine you have.
Need to know what year and what engine to help you.
Assuming this is the 4.3L engine, the distributor cap would be found in the back of the engine on top. All the spark plug wires lead to it.
On the 4.3L V6 the distributor is located in the back of the engine, in the middle. Just follow the spark plug wires and they will lead you to it.
This sensor is at top rear of engine, next to distributor.
electronic spark timing is your distributor.
Easy....Read the manual.
The engine is not hitting on all cylinders or the distributor (electronic or manual) needs adjustment.