how to modify the equation in Fluent
 

hello everyone
I want to modify the momentum equation.
The first item and second item on the left side of equation have sigma and sigma^2, respectively. By the way, the sigma is variable instead of constant.
What should I do can modify the equation.
Thank.
http://chuantu.xyz/t6/702/1571282009x992245926.jpg

I know the udf can be used as a source term (like Su (the last item on the right side of equation)), but how to change the left side of equation.