There's any number of ways to do it, but the most important thing is safety. NEVER have your body, limbs or fingers between the rollers and the boat hull. Use the framing of the trailer as your protection should the boat fall back down onto the trailer. With that being said, however, as long as you take your time and think things through, there really isn't any chance of it falling on you. I'm probably making this sound a bit scary, but it really isn't so bad. To give you an idea - when I was 15, I decided to take our boat off the trailer and put it on blocks in our driveway... just for the heck of it. My dad came home and asked me why I did that. I told him I just wanted to see if I could do it. He thought about it for a minute and then shrugged his shoulders and said "OK, just make sure you put it back on". Point is, if a 15-year old can do it...
You didn't mention if the roller is towards the front or the rear, but release the bow strap and stern stern straps. You'll then do the jacking under the keel, either near the transom or just aft of where the bow keel starts to turn up - depending on which is closer to the roller in question. In addition to your floor jack, you'll want a couple pieces of 2x to protect the keel (keep the graining perpendicular to the keel. You should also use a jack stand instead of relying on just the floor jack.
Jack up the boat till it rises off the rollers a bit (you'll first be unweighting the suspension). Place the jack stand, but leave the floor jack in place, too. It may rise more on the other side as the boat will likely "list". If it does, just push it over to list the other way. Tie the boat from a cleat to the trailer to keep it leaned over. Now put some more pieces of wood in between the hull and roller arms (secondary precaution, doing basically the same thing as the ropes). Remove pin and washer, replace roller.
Before you start the project, closely inspect the others and replace all that are "iffy" - they're inexpensive.
