Undo the 2 hose clamps holding it on and pull it out, if its been on there for a long time it could be pretty stubborn. I suppose do it outside/over a bucket as a bit of fuel will come out and then if fuel keeps pouriing out tie up the hose higher than the tank to stop it.
Buy a new one from repco etc for about $5/$10 is how you clean or service it
. Cut it apart and seeing if there is any muck in it would be a good learning exercise.Some sierra tanks do have a nut on the bottom so you could drain them. Poke your head under and have a look?, Otherwise its disconnect it all and drop it out and pour it out the fillup or fuel level hole etc. You really shouldn't have to drain the tank unless your modifying it or it has mud in it etc as its a horrible job, check that filter first!
Cheers!