To modify clock frequencies you have a couple of options depending on which signal you want to utilize. You can configure and enable usage of the PLL or FLL in registers pertaining to the MCG (Multipurpose Clock Generator), or clock dividers in registers for the SIM (System Integration Module), or just simply change the source of your clock signal to a different reference clock. These are all ways to alter the frequency.
I would recommend using the API as it can make life a lot easier. I discussed the concepts required for setup of the clocking system in the following link, as well as talking about where to find the API and what you need to change to use it for your application:
How to use internal clock for K64F
While the discussion was for the FRDM-K64F board, it should be the same for your board assuming you are using SDK 2.0.
