How can I push then back inside a little?

 

Use a big C clamp or large pliers

 

Crack the bleeder screw, and use a c-clamp to push them in.

 

A bock of wood across the pistons (1/2"x 2"x5" dimension or something similar) and a clamp from the top side of the wood tightening down on the backside of the pistons. Maybe a 4" clamp or 5" would work? This is what I always do.

 

Correct. But take an old pad and put it over the pistons, this way you push both in at once and wont push one out or bind it up or whatever. Also watch your fluid level in the resivior while doing this.

 

You can mount the caliper back on the spindle without pads (finger tight on the bolts) and slide the pistons next to the rotor, small prybar or big screwdriver and pry the caliper toward you.

 

Be sure and watch the level of brake fluid in the master cylinder. You may have to suck some out as you push the pistons in. You can also pinch off the brake line and run the excess out the bleeder line.

 

What happens if piston gets a little damage? I tried it with my old old ones don't worry

 

Don't pinch off a brake line, you could damage it internally, opening the bleeder valve works best and it keep any dust build up in the caliper from going back up into the master cylinder, just rinse any spilled fluid with water.