Um... Ok I feel completely inept and retarded here, but I cannot see an actual "google wallet" app. It's neither on my phone nor on my tablet. Is it called something else?
Oh my goodness, I took the silliest route imaginable, lol:
I bought an app, and when I got prompted to pay, I was able to amend my CC info. I'm sure there's a way to do it that doesn't require to spend money in order to do that, but since I couldn't figure it out... My pragmatic side took over. Good thing I didn't need to use the same method to edit my car payment options, that could have been VERY costly.