How do I go about applying for British passport renewal for my 86?
There is no such thing as Nigerian British citizenship anymore. It stopped being a thing when Nigeria became independent. if she remained in the Nigeria, and the timings suggest she may well have done, then she have will become a Nigerian citizen but stopped being a British one. So its possible that she will need a tourist Visa. If, however, the question meant that she has dual citizenship and is a British citizen and a Nigerian citizen then she could just renew her passport at the high commission.