As in, you've got to heat up the glass enough to neutralise the glue holding it in place, then prise it away from the casing without cracking it. Once that's off you've got to undo the metal plate holding the flex cable to the logic board, and then disconnect the battery from underneath the display whilst making sure the display itself doesn't become disconnected first (because if it does, a fuse leading to the backlight will blow and you've got a rather interesting soldering job ahead of you).
If you're not familiar with heatgun usage and so on, forget about replacing the battery yourself.
Bear in mind that in some regions (eg Australia), local laws may grant you additional coverage on top of your warranty duration.
On the flip side, batteries wear out through use, and if yours has gone through a certain number of charge cycles warranty won't apply even if you're still within the coverage period.
In any case, it wouldn't hurt to get an Apple authorised service provider to check the device out.
