Put your #1 cylinder at top dead center (procedure is easily found on Internet) and then note the position of the old distributor rotor. Remove the hold down and then gently lift the old unit out. The rotor will rotate slightly to the left as you remove it due to the gear. Remember this as the rotor will rotate slightly to the right as you install the new one!
Once installed, check the timing with a timing light to be sure you have it installed correctly.