Put the hammer at half cock, so the cylinder rotates freely. On the right side ahead of the cylinder, there is a big slotted screw head. With a screw driver, turn it about 90 degrees counter clockwise you'll hit a 'stop'. It should move easily. Then you can pull the the lever and base pin assembly out ((the screw head pin doesn't come out, it just turns one way or the other)). Before I remove the base pin, I rotate the cylinder so as you move the loading lever down it pushes against the cylinder, which helps to pull the base-pin a bit more easily if really dirty. Quite simple. Reverse steps to replace. Should try this before you shoot, so you know how it all works. Oh, yeah, hold on to the cylinder as once the base pin is free the cylinder will fall out the side.
{rereading} Oh, you want to remove the retaining pin. Don't think in all my years I've ever removed that from any of my ROAs. Check you manual as stated above....