Increasing PHP Memory Limit for high end theme

I need to increase my memory from 40mb to 64mb. My host just updated my memory to 128mb but i just read WP memory is different than host memory. All the instructions make no sense to me since I didn’t install the theme myself. I paid someone to install it and woo commerce. Can someone either do it for me or tell me how to do it in a STEP BY STEP beginner way?