If you don't have a lot of experience doing this type of operation I would not attempt this. Since you are asking the question it's safe to assume that you don't. Fire control parts in the AR are surface hardened only. It is extremely easy to polish through or thin the surface hardening to the point your trigger will no longer function safely.
In your situation I would separate the upper and lower. Take the lower and dry fire it at least 1000 times catching the hammer with the palm of your hand. This will smooth the parts without removing the surface hardening. Disassemble and clean the fire control parts, install your JP yellow spring, apply moly lube spray or moly grease then reinstall. You will have a much smoother trigger.
