+1 on the overfilled master cylinder. You probably topped off master cylinder before you changed pads. Now there's too much in there, not allowing the pads to retract enough.
To disassemble the pistons is quite easy. Take the wheel off, then squeeze the brake lever. That'll pop one of them out, making a big mess in the meantime! The other can be gotten out by refilling the master cylinder and blocking off the first side and squeezing again!
Now, in all seriosity, the piston can be pulled out fairly easily with a pair of water pump pliers, er, ChannelLocks. Still makes a mess when the fluid comes out. It should be necessary though. You might want to clean the gap in the bore with the pistons in with some brake cleaner. If the gaps get a lot of crud in them, that can keep the pistons from retracting.
Bare