What needs to be done to a central ac unit if it blows, but only mild temp. air, and the inside unit has just been replaced? If it needs freon how much for a 2 ton and where can I buy? And price? Help!

Adding freon yourself is not possible. You need to be EPA certified. Need to call in a tech for service.

You can not purchase refrigerant without being EPA certified, call a pro, much cheaper in the long run. It does sound like you may be low on refrigerant. Is it possible that they didn't properly charge the system when the replaced the indoor unit?