Do you mean the piston came out of the caliper? How did that happen? If the pads were grinding metal-on-metal, you have bigger problems. The brake rotor will need to be replaced. Since it's becoming more common on trucks to develop a brake pull from unequal rotor thickness on the two sides, the best is to replace both rotors. It takes a long time of ignoring the grinding brakes for that to happen, and by that time metal grindings will likely have found their way into the caliper so it's best to replace both of them for even braking.
If the piston simply came out too far from improper repair procedures, it can be pushed back in. Ford pistons usually push hard so you might want to open the bleeder screw to make it easier. If the piston came out so far that it's past the dust boot, you'll need a special pliers to open up the boot, or you'll have to work on it on a workbench with compressed air.
Since you didn't provide any details, I have to wonder if all you mean is the piston is part-way out because the old pads were worn down and you're installing new ones. That is the self-adjusting feature of all disc brakes and pushing the piston back in is a normal part of the service. Most professionals pry them back in with a screwdriver before they unbolt the caliper. If that doesn't work, there is crud built up inside it and it will most often cause brake sticking. We used to always take them apart to clean them and rebuild them with new seals and dust boots, but today rebuilt calipers are so inexpensive that we just replace them. If the pistons DO slide back in easily, that's all that is needed to make room for the new, thicker pads. Some people believe you must use a c-clamp to force the pistons back in. While that does work, if you MUST use one to get the pistons to move, they are not moving freely and the caliper must be replaced or rebuilt.