Step-1: Use a blower brush to remove dust and dirt, in case you don't have a blower you can also use an ear syringe. Never blow on the lens with your mouth, as it increases the risk of accidentally spraying the lens with saliva.
Step-2: The basic thing to clean the lens is that take a microfiber cloth or a lens cloth and clean the lens using the clothes. In case, if it doesn't work put a drop of lens cleaner on the cloth and try again.
Step-3: If you wanna clean dirt inside just remove the back cover of your phone an put on small amount of alcohol-based liquid lens cleaner and a Q-Tip cotton swab to clean lens from inside the back cover. Its a bit risky.
At the last I'll just say clean your lens as seldom as possible. Must share your experience after cleaning your phone’s camera lens via your comments.