the way I was taught, u are supposed to keep your thumb toward the bottom of the neck so you can reach with your fingers easier, but instead I rest it between my thumb and index finger, that gives you all the control you need, but the only real ways to get good at the changes is to find a song that uses em and practice a minimum of an hour everyday. If you're really dedicated and wanna learn, you'll get it. A good song to practice is that leave your lights on song by everclear and santana, the rythym part goes, Am,C,G,F,E

it sounds cool and is a good practice for your C to G change. Good Luck