Installing a new tank is fairly simple, but you need a big pair of pliers or a special wrench to remove the flapper/overflow assembly. You need to remove the overflow/flapper valve assembly from the old tank (unless you buy a new one - the tank might come with it, but it might not), and the fill valve. Then unscrew the flush handle and arm/chain. Reassemble the parts into the new tank, and bolt it to the bowl. Normally, there are two bolts, some have three. You probably want to get new washers to assemble the thing to the bowl. If the shape of the new tank is identical, then you may not have to adjust the height of the water in the tank. If it isn't, you need to adjust it so you'll get a good flush.
Depending on the cost of the new tank, and if you have tp pay someone to do this for you, it might be about the same price to just buy a new toilet altogether. Often, parts end up costing more than the factory assembled new.
