If you are going to set your own value for say transmit power at the start of the simulation itself, then you can update the values in the init function itself (fn_NetSim_CR_Init), which is part of the 802_22.c file. 


If you wish to modify during the simulation then you can access the desired variable in any of the functions and suitably modify.


You can also add your customization either in a separate C file which can be added to the CognitiveRadio project or in any of the existing C files that are already part of the CognitiveRadio project.


For reference, you may look at the function fn_NetSim_CR_AssociateCPE() which is part of the SpectrumManagar.c file where some of the CRCPE MAC and PHY variables are updated during runtime.