You can just rebuild the bad one, but you should give the other one a good inspection. You might want to check to see if there's seals available for the calipers so you can disassemble the bad one. If so, take it apart and take steel wool to the piston to clean off any rust, clean out the other parts, and apply a piston lubricant if the manual calls for it.
You likely have a split master cylinder that separates the front and rear brakes, in which case you could just bleed the brakes on the front. If it's reached a point where the caliper has gone, though, you might want to bleed all the brakes to flush out the old fluid and replace with new.