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Before I get into how to replace it, I should ask, does it really need to be replaced? 
If it is leaking, then it's simple, It needs to be replaced. See Boiler relief valve
If it is full of water, it needs to be replaced, Tap the tank the back should sound hollow, if not, it is full. if it is full, that means it will be very heavy, So be careful not to let drop on your fingers when you remove it.

The Rest is easy unscrew the old one and put some kind of pipe dope or tape dope on the threads. (only on the315 and #30) and screw the new one back in The #109 And #110 do not need and pipe dope or tape dope they have an O ring that makes the water tight seal. ok that's it you are done put the auto vent back. turn the boiler drain off turn the water back on Get the Air out turn the power back on.

Furnace Expansion tank 15