Pls How can I make 16F877A use 15MHz Xtal

[T.Jackson]

Hi I am using a PIC 16F877A in my project. The Project works fine with a 4MHz crystal but when I replace the 4MHz crystal and its associated capacitors with a 15 MHz crystal and associated capacitors, The pic does not work
Does anyone know what needs to be done in PIC basic to make the 16F877A work?
Thanks

There is actually a variant of the 877a that is limited to 4MHz. 877a-20p will work up to 20MHz