New to this "game"

I toyed around with it a few years ago, and still have 60 credits.  How do I make it a premium account by using credits in my balance because it isn't automatically spending them.

http://en.airlinesim.aero/wiki/index.php/Tutorial/1.5/general/Payment_System

And those are free credits you always have when returning. You need to pay for credits and your account will automatically updated to premium. Once it is, it always will be