Replacing brake calipers is not a simple procedure, even on a small car like the Ford Focus. It's always best to have a trained professional handle such maintenance. If you do try to replace a caliper on your Ford, make sure you have all the parts and knowledge for such a task.
Lift the car off the ground on a jack stand. Remove the tire and wheel.
Disconnect the brake hose from the support bracket and clamp or plug it. Remove the outer brake pad retaining clip.
Remove the caliper from the steering knuckle. Disconnect the bolts, pivot the caliper upward from the rotor and slide it off the pin. Remove the brake pads.
Detach the brake hose from the caliper at the mounting bolt. Toss away the washers on the bolt.
Attach the brake hose to the new caliper. Tighten the fitting to 11 foot pounds. Attach the brake pads.
Install the caliper to the steering knuckle, using the reverse method of removal. Tighten the pin bolts to 21 foot pounds for a front caliper and 26 foot pounds for the rear.
Connect the brake pad retaining clip and the brake hose to the support bracket. Install the wheel back on the vehicle.
Bleed the brake system to remove any air. Lower the vehicle. Test the brakes, first while stopped and then on the road.
It's a good idea to disconnect the negative battery cable and remove two-thirds of the brake fluid from the master cylinder before removing the caliper. Remember to refill the cylinder after installation.