defiance

I had my 06 sixer auto tranny rebuilt half a year ago, had a shop go ahead and put a shift kit in it. Shifting is pretty firm and at first i liked it, but it also made it down shift firm. Well I am finally tired of it kicking the whole car when it down shifts, so now I need some advice on what to do. I have read about being able to adjust the shift firmness by adjusting line pressure, but i dont even know where to start, can any one help me out?
Thanks in advance.