Big Mike already explained it pretty well, bu if youre still having problems, i might be able to describe it better to you because i use PSP myself.
assuming you have the original shot and the infrared edited one, open both in PSP.
right click on the infrared version and click copy.
right click on the original, and click paste as new layer.
make sure the infrared layer is highlighted in the layers toolbar on the right side.
click the eraser tool and set the size to be about half the size of the area you want change, in this case, the eyes. set the hardness to between 50 and 75.
start erasing the eyes on the infrared (top) layer and youll see the original eyes come through.
once your satisfied that it looks the way you want it to, click "layers" on the top toolbar, then go to merge, then click merge all / flatten.
save the final picture when youre happy with it.
remember too, when erasing, if you want or need to unerase a part of the picture, just right click to unerase and go back over what you didnt want erased.
if you need any more help with PSP, just let me know. ive been using it for a long time, so im pretty familiar with all the tools it has.