Replacing a CenterLock disc is piece of cake. But before you get down to bussiness, you must identify wether your disc uses a CenterLock or 6-bolt system to get mounted on the hubs.
For completing this operation, you will require a cassette tool especially designed for unmounting the sprockets, as the one pictured below. It can be bought from companies such as Shimano or Park Tools for a price of around 10 euros.
Mount the cassette tool on the lockring.
Using a wrench, loosen the lockring until you can remove it completely, by turning the wrench in the movement direction of the wheel.
Remove the rotor.
Mount the new disc in the designated position.
Place the cassette tool in the lockring and, by hand, screw it in.
In the end, tighten the lockring according to the manufacturer’s recommandations, which can usually be found written on the outer part of the lockring.