You will need to move the piston out of the cylinder. The dust cover (gaiter) fits into a recess on the piston so it can be stretched quite easily for removal and replacement.
Ideally, you should remove the calliper from the vehicle. Push the piston back with a 'G' clamp to remove brake fluid, then carefully apply air pressure to push the piston back out. This helps keep everything dust free.
A side of the road method is to unbolt the calliper with brake hose still attached, remove brake pads, then gently push on brake pedal. The piston will be pushed out far enough for you to proceed as above.