In the air force can you change your job if you have already signed a contract for it and are still in the DEP?

If you've already done your contract then no you can't. If you refuse the job, they will release you from the DEP. I've been recruiting for the Air Force a little over three years and I hate to shatter your ego, but I guarantee there is someone who is willing to take whatever job you have. The job on your contract is one the 5 you listed at MEPS that YOU said you would do. So the Air Force isn't going to bend just because you changed your mind.