How hard is it to change the donut seal under rooftop AC?

I've had to replace those seals twice in the past, and it's just as others have said. First see if you can find the leak, check the drain, then tighten the screws/bolts. If you still have a leak, it's not all that difficult to replace the seal, just some grunt work. Best to have someone helping you lift, and have an old mover's blanket or something like it to set the AC on (or a third person so you can put the new seal on while two hold the AC - just be quick!).