There is a guide here:
Be advised that you need one or more Torx screwdrivers (at least n.6 but for the harddisk also n.7 and 8). Some battery vendors sell a replacement battery with included Torx screwdriver.
When you start this operation, it is a hell of an opportunity to also replace the 2,5 GB Seagate Microdisk with a 4 or 8 GB CompactFlash card.
- The 2,5 GB drive essentially is a hard disk with moving parts. It can break down, it is slow and it is not that big any more these days.
A modern CompactFlash card is faster (get a 133x or faster one), bigger and consumes less energy because of the lack of moving parts. See it as a Solid state disk upgrade to your TomTom.
The most used card for this would be a 4 GB Transcend 133x card and here are the instructions: