Answer:
If you consider doing this the first thing is SHUT THE MAIN BREAKER OFF. Use an auxiliary light source to see when working in a dead panel. Always keep in mind an ARC FLASH ACROSS HOT MAINS HAS ENOUGH HEAT TO PEAL THE SKIN OFF OF YOU. Before an explanation of how to do it you should keep in mind that this is for a single receptacle only. If any other receptacles are on the same circuit they will also be changed to 240 volt receptacles and if you plug any 120 volt device into them, the 120 volt device will be destroyed. That said OK, any 120 volt outlet can be changed to 240 volt by removing the 120 volt circuits neutral, re identifying the white wire with red or black phase tape and connecting it to a new 2 pole 15 amp breaker. The maximum that this circuit will now load up to is, on #14 wire at 80% = 12 amps at 240 volts. The 120V receptacle must be changed to a 15A - 240V receptacle so that 120 volt devices can not be plugged into it. Never use 2 single pole breakers together because code requires that on a fault trip both legs of the 240V circuit must be disconnected simultaneously. If you have any questions use my message board.