I have a iPad Air model ME999LL/B And software 12.4.9 can I upgrade software

No. That is a first generation iPad Air (cellular) and iOS 12.4.9 is the highest iOS version it can run.

iOS 12.4.9 was just released a few weeks ago for devices not capable of running iOS 13 or later. It is up to date for those models.

You can trade it in, and pay the difference for a new one yes.

See here: Apple Trade In - Apple