absolutely nothing! I find it adorable when a dog has one ear up and one ear down. most people find it cute. why do you feel the need to do something? the ear will probably go up on its own, eventually. But even if it doesnt, there is no need to do anything. As long as your dog is happy and healthy it is great! (PS my akita has one ear up and one down, and people stop me all the time to tell me how cute it is!)
PS do not dock or have your dog's ears cut. it is inhumane and unnecessary.