There different methods you can use to convert your Bitcoins to Naira. You can either do it by selling it through a payment system that accept Bitcoins and gives you the cash equivalent in Naira. The 2 payment system i have used before for such transaction are Luno and My patricia.
However, note that their rate are different and the processing charges are also different. You can download their app from google playstore or on IOS store also.