Check out page 64 of the datasheet. It will tell you how to set the internal oscillator frequency.

To set the oscillator to 8Mhz for example:

Code:
define osc 8
OSCON=%01110000
or 4Mhz:

Code:
define osc 4
OSCON=%01100000
*edit* Page 64 according to adobe anyway - page 62 according to the page numbers at the bottom of the pages.