Depends on your level of expertise ;)
I'm assuming it's not too dissimilar from the series 3 tank, so I might be wrong..
The tank is held in by either three or four bolts, three at the front (holding it onto a chassis outrigger) and one at the back which has a strange rubber grommit affair.
The first thing you should do is take the front seat out, or apart. If tilt it forward you should be able to get at the rubber filler pipe and remove it from there (loosen the jubilee clip). There is also an access panel for the remaining pipes (the feed) and the wires for the fuel guage, these should be removed, and should be easy enough to do.
The ones at the front will likely be rusted and beaten to something unpleasent (they're in direct line of fire for mud and grime off the front wheel), the rear may well be similarly unhappy. For the cost (a couple of quid) get a new set with the new tank.
With these removed, and a suitable jack or similar underneath you should be able to carefully lower the tank. It's not *that* heavy, but the positioning will make it a painfull job so think carefully about how you're planning on doing it before you do so.
hint: empty it first.
Fitting the new one is, as they say "the reverse of removal", except you swear in different places.
Basically, I'd have a poke about, drop the seat forward etc and make sure you're comfortable with where things are before you start, and it should be easy.
