I wanted to upgrade my Macbook Pro's SSD from 128gb to 256gb.
I followed iFixit's step by step instructions, and everything went smoothly.
I know it might be a little scary doing this the first time (It was for me). Don't worry to much about hurting the machine though. Macs are built strong and are built to last.
T5 Torx Screwdriver
P5 Pentalobe Screwdriver Retina MacBook Pro and Air
TR7 Torx Security Screwdriver