There are couple of ways to do this but both are post production treatments.
If this was mine, I would simply take the .wav files from the device to my computer and load them on the Vegas timeline and adjust the balance there.
If you don't have Vegas, there is a free audio recording software called "Audacity."
Once you install this free download, then load your .wav file to the timeline and adjust the pan/balance (L to R) control on the left of the timeline.
Too bad you did not notice this when it was still under warranty, but then again maybe all Zoom H2 do this?
In addition to this I would recommend the following troubleshooting tests to find out where the problem is being introduced.
1) Is the unbalance evident on all the H2 "Mic Patterns"?
Read pages 14 through 21 (H2 manual) and test the different mic patterns, are they all unbalanced?
if you lost the manual: download here:
2) Are the built in mics balanced?
to test this, borrow a stereo mic that you know is well balanced and plug it into the external mic input to the H2. Do you still have the unbalanced? if so the problem is not the internal mic, it has to do with internal mixer or output. If using an external mic improves the balance than there is the solution: the internal mics are unbalanced and you should use an external mic.
3) How to test the internal mixer and output. Since the H2 has a stereo line input, then get a CD player that you know has a balanced output with balanced output CD, record this with your H2. CD-->H2. Is the output of this test balanced? If yes then there is an issue with the internal mic preamp balance. If not, then you still have some sort of output unbalance issue.
4) Are your headphones balanced? Check your headphone with other media output to see if these are balanced. Maybe the problem is with your headphones?