Replace a piston ring.
Help replacing a piston ring.
Had I replaced the piston ring on my compressor, and it just happened to be the same brand as yours, then I would be able to provide you with a step-by-step method.
You didn't indicate what brand of compressor you have so even if it was the same as mine, that option is entirely out.
The replacing of the compressor piston ring involves special pliers, usually ones that expand the jaws when the handles are compressed. The tips are bent to fit into holes on the ring, and when the handles are compressed, the jaws open, prying the ring apart and allowing it to be pulled out of the piston groove.
What I think you're really after is a tear-down and reassemble process to get to your piston and the piston ring, not just the how-to of replacing the ring itself.
You know there are thousands of types and sizes of air compressors, and although there will be similarities in each, each will be sufficiently different that no step-by-step methodology will work for all.
If you have the manual, you'll have in it (at least there is normally) an exploded view of your compressor. That's the information and guide you'll want to use to start tearing down yours.
If you don't have the manual, ask on the need a manual page.
If you have the manual, and the exploded diagram of your compressor intimidates you, then perhaps you should consider getting the unit repaired by pros?