How to rewire 110v socket to be 220v

More info, please! If you are talking about a standard wall socket, you could kill someone. Hopefully, you are are talking about a dryer, etc type of socket. Please repost with more details.

If it was going to a 120 volt wall receptacle for a 120 volt AC and you installed a 240 volt ac and the wire was #12, and the 240 volt AC required a 20 amp circuit, you could remove wire from neutral bar, identify it with black tape and put on your 240 volt 20 amp 2 pole breaker, and at the receptacle, switch to 240 volt receptacle with the white wire reidentified with black tape.
This would HAVE to be a lone receptacle, and the neutral can NOT be part of a multicircuit.

Your best bet is to run a new circuit. It is unlikely that there is nothing else on the existing circuit.