You can (generally) press the pistons back into their bores by using a screwdriver as a lever against the edge of the disc w/the tip on the center of the piston.
There's also specialty type tools that slip in and jack both pistons back at once with the turn of a knob/handle.
Anyway, if you have to remove the caliper, there's two options:
1) undo the hardline at the caliper and catch the drips, then do the bleed thing upon reassembly.
2) looking at the backside of the hub, you'll see that the hardline has a bracket affixed to it that ties it to the hub via a nut and bolt arrangement. Remove the nut and the hardline will pop off the back of the hub and let you flop things about on the softline. (upside is it's easier as you don't have to bleed it afterwards, downside is sometimes the bolt that sticks out will displace into the back of the hub behind the disc rotor forcing a teardown of the hub and wheelbearings to get it put back in place)
If your rotors aren't coming off to be turned or otherwise serviced, and it's just puck replacement you're after, I'd suggest just leaving the caliper in place and levering the pistons back one at a time, putting the new puck in, then repeating on the other piston.
There's also specialty type tools that slip in and jack both pistons back at once with the turn of a knob/handle.
Anyway, if you have to remove the caliper, there's two options:
1) undo the hardline at the caliper and catch the drips, then do the bleed thing upon reassembly.
2) looking at the backside of the hub, you'll see that the hardline has a bracket affixed to it that ties it to the hub via a nut and bolt arrangement. Remove the nut and the hardline will pop off the back of the hub and let you flop things about on the softline. (upside is it's easier as you don't have to bleed it afterwards, downside is sometimes the bolt that sticks out will displace into the back of the hub behind the disc rotor forcing a teardown of the hub and wheelbearings to get it put back in place)
If your rotors aren't coming off to be turned or otherwise serviced, and it's just puck replacement you're after, I'd suggest just leaving the caliper in place and levering the pistons back one at a time, putting the new puck in, then repeating on the other piston.