How to change it depends on your needs. If you often ride a flat road, I suggest you change to a small sprocket, which is faster and more fuel-efficient. If you often climb with heavy load, I suggest changing to a big sprocket for more power and easier climbing. When replacing, the front and rear chain discs and chains are usually replaced together to avoid the old and new being mixed together and easy to wear. When I say chain wheel, I mean the one on the rear wheel. )