crack the case open, you will see a bunch of cells soldered together, replace reseal

Its a pretty easy practice, you just have to make sure you get the right kind of batteries and that they can provide enough (or more) amperage

and physically they are often the size of AA batteries in these bricks, maybe C sized or D sized in the tube style batteries

Here is Kip Kay on the subject, course this guy can make the flux capacitor seem stupid simple

http://kipkay.com/videos/battery-hacks/laptop-battery-hack