OK, for starters
@natureboygrower is correct, the wire needs to be upgraded to carry the additional current.
What can be done and I don't recommend it is changing a 15 amp 120 volt circuit into a 15 amp 240 volt circuit. This is for lighting use only. Only attempt this if you have a good understanding of electrical and the proper safety procedures, it's not worth dying, hire an electrician and just have a new feed ran. You can only do this once the circuit in question has been checked and verified that only receptacles are on that circuit (no lighting) I have seen people fry their microwave thats hardwired above the stove doing this.
Turn off the breaker for that circuit.
Pull the breaker and disconnect the black wire.
Disconnect the neutral wire (white) for that feed from the neutral bus in the panel.
Put a black or red electrical tape on the insulation by the end to indicate this is now a hot conductor.
Using a double pole breaker of the same amperage put both wires on it and insert into the panel.
Now those receptacles are 240 volt and should be marked to prevent you from plugging in a 120v item and frying it. Of course now you have to find an alternate source for your 120v power. lol It can be a fix in some situations but like I said, I don't recommend it.