I have a GE12k BTU air conditioner using 110v, and I have a prewired 220v air conditioner outlet available. So how can I rewire my air conditioner to use my 220v outlet?

Not worth the cost. Sell the 120 and buy a 240.

You can change the 220 volt to a 115 volt outlet but you will need to know about how to do electric wiring and the wire size the 120 unit needs. It is cheaper than buying another unit.

Hvac 1000, the supplies I have are:
12/2 copper wire, 15a outlet, and breaker box with 20a double pole spot to rewire the outlet for the air conditioner. I can replace the breaker with 2 15a single pole breakers or should I upgrade the outlet to 20a and 2 20a single pole breakers?

On the front or side of the window unit after you remove the cover and pull the unit out of its case will give the amp rating. Max protection amps and min protection amps.(go with the max). Its as simple as removing one line voltage wire going to outlet to the common bar on your breaker box(tape off jacket with white tape to cover the black or red color wire). To represent common. Remove the 2 pole breaker and install a single pole breaker rated for your system. ALL WORK SHOULD BE DONE FROM SOME ONE THAT KNOWS ELECTRICAL.