Until they go wrong you’d be forgiven for not giving your jockey wheels a second thought (unless you’ve upgraded to some fancy oversized ones for bonus bling points). They sit out of sight, doggedly keeping your chain in line day in day out, but if you’re regularly exposing them to muck you can slowly wear them down to the point of failure. They should have a bit of play in them to account for changing chain lines, but anything more and you risk losing your chain between the jockey wheel and the derailleur cage, which is something that's happened to me recently.
Servicing the wheels isn’t a difficult job at all, and requires no more than the right sized hex key, a good clean and some fresh grease, but it is one of those jobs where it pays to be organised. It’s also a job that is substantially easier to do off the bike, so undo that quick link and remove the whole derailleur. While some jockey wheels are interchangeable upper and lower, some are not, and some are unidirectional too. I find it best to have a small tray, or our Magnetic Parts Bowl to keep the parts safe when you remove them.
Unscrew both derailleur cage bolts and release the wheels, keeping them in order. Give everything a good clean with some degreaser, including the cage plates. Leave them to dry thoroughly then apply a liberal coating of grease to the wheels and the metal cups. There will likely be little metal bushing in the centre which may or may not come out. If it does, grease this too. Now it should be a simple case of reinstalling the wheels in the orientation they came out in.
Note - the upper screw may be difficult to access if the derailleur body is in the way, but this can be overcome by applying some elbow grease.