Treat it with an antiseptic, and try identifying, and correcting the problem that caused the infection in the first place. After the infected part of the shell is clean of debris, and dry use antiseptic to kill bacteria in the wound. API Turtle Fix is a good product to use. If you can’t get Turtle Fix, or a similar product for reptiles diluted betadine solution is safe to use. Keep in mind you want to let the antiseptic dry on the wound before returning it to it’s regular enclosure. Repeat daily.
Now what caused the infection?
Do you recall the shell getting damaged in anyway?
What’s the enclosure like that you’re keeping it in?
Is there a land area big enough for you’re turtle to get itself completely out of water? This is necessary in order for it’s shell to dry out.