Change the Sub_Total Value

Dear Concern,

I have paid version of this plugin.
I need to change the functionality of sub_total; I want to modify this “Sub_Total” functionality, where can i find in code, means in which file?
or
How can i create new variable that calculate different values.
Normally sub_total is calculate the sum of all (item.Price) * (item.qty). but i need to modify it.