I'm assume you're not working on a polyphasic system, so you have 2 hots and one neutral with 220VAC across the hots and 120VAC across either hot and neutral:
The motor should have two sets of windings inside. The leads for these windings should be behind a cover plate or in a junction box. Since the motor is currently wired for 115V, the windings should be in parallel. Write down which wires are connected to what. This is very important in case you make a mistake!
A good way to think about this is you have four wires, numbered 1 - 4. Winding one has two leads, 1 and 2. Winding 2 has two leads, 3 and 4. Right now 1 and 3 are connected to the hot, and 2 and 4 are connected to the neutral. To wire the motor in series you want 1 connected to hot1, 2 connected to 3, and 4 connected to hot2.
-EDIT- There was a mistake in my first revision. The method wouldn't work for 110-220 conversion as a mistake could burn up a winding. -TJNII
First, figure out which wires are winding pairs. This is very important as if you try to rewire it blind, you could burn a winding out. After writing down which wires were connected where, disconnect all junctions. Connect your multimeter (on resistance) to one of the wires you disconnected from the hot. Find out which wire from the neutral is paired with it. The matching lead will have a fairly low resistance. You know know which leads are 1 - 4. Wires 1 and 2 are the pair you just found, and wires 3 - 4 are the pair you didn't need to test. Wires 1 and 2 connected to the hot, and wires 3 and 4 connected to the neutral.
Connect 1 to hot1, 2 to 3, and 4 to hot2. Check the resistance across 1 and 4. If it is not about double the restance you found in the above step, you made a mistake. Re-check your wiring. The motor should now run.
If the motor tries to turn but cannot, or turns slowly while humming very loudly, wires 1 and 2 are reversed.
Do not run the motor with the windings in parallel, or run a single winding off 220V! You will burn it out!
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