To install seals on twin piston calipers you need air pressure. You will have to split the caliper and use air to pop the pistons out of the caliper halves.(Use eye protection and cover the caliper with a rag) then, with out scratching anything, remove the seal from the caliper body Check for any wear or damage(It is unusual for seals, even old ones to just leak). Clean and dry all the parts. Use the correct brake fluid (mineral or dot 3/4) to lube and install the seal in the caliper half. Reasemble the caliper. Install and bleed out the system. If this is unfamilar to you find a competent shop to do the job unless you like having no brakes.