Systick Clock Source

I am using CCS5 Version: 5.4.0.00091.  I am using a EK-LM4F120XL launchpad with a LM4F120H5QR processor.  I have written a program using systick commands and the program works great.  It uses the system clock because bit 2 of the NVIC_ST_CTRL_R is set.  I have the following commands which alter Systick interupt and Systick enable or disable.  What is the command to change the clock source bit from a 1 to a 0. 

SysTickIntDisable(); //Not using Systick interupt so turn it off.
SysTickEnable(); //Enable Systick

I am 64 and live in south Texas so any responses should be written slowly and very loud.