On many of the newer Dell laptops, you have to remove the bottom cover. Dell usually uses ~4 screws to secure the battery. Once you remove all of the screws that hold the pack in, disconnect it and replace it.

Refer to this manual (online edition) to remove the battery.

To show you how to do it on most of these new Dells, I tore apart a 7490 to give you an idea (yes, this IS one of the 2nd HD or big battery models). Sometimes sticking a guitar pick in a corner of your choice as leverage can he helpful as well. These are not my best photos - I take more care in guides; this is more to give you a starting point from someone who has dealt with these bottom clipped laptops in general enough to know a few tricks.