Piece of cake, assuming the valves hold.
Just buy a whole new float valve, they're cheap enough not to worry about just replacing bits of it. It'll come with the correct washers, or it should do anyway: If not, ask your merchant to give them to you as well. Simply isolate the water supply, then using a couple of adjustable spanners or grips or whatever you have to hand, undo the nut connecting the feed pipe to the valve. Careful when you remove it, as there will likely be a small red fibre washer you really don't want to lose! If you want to get a spare, not the worst idea, it's called a tap connector fibre washer.
Then using the spanners, undo the two nuts either side of the tank wall holding the valve in place. Pull it out, noting which side the washer/s is/are (one is fine, should be the wet side of the wall, some plumbers like to use two so one either side, neither is wrong), then simply reassemble the new one in exactly the same way. You want the nuts holding the valve in place to be quite right, but assuming they're plastic don't go mad as you can strip the threads.
Do the nut up on the feed pipe, remembering that little red washer, then turn back on. You'll almost certainly have to adjust the valve to get the correct water height in the tank, this may be done either at the end of the arm near the ball OR at the end nearest the tank wall. Either way it'll be two nuts, one to adjust and one to lock, it's very obvious what you'll need to do.
If you want to grab a pic of the old and the new, I can talk you through it more accurately if you like.