Every nokia phone that I have ever had or seen has had a removable, replaceable battery and the n8 is no different and while it may not have a easy sap off back like other nokia device, there is  removable battery and this is how you get it out

Unscrew the Torx+ size 4 screw on each side of the Nokia N8 7 full turns. Do not remove the screw completely!

2. Pull out the bottom assembly.

3. Lift and pull out the battery lid.

4. Use the pull-out tape to remove the ORIGINAL battery.

5. Attach the pull-out adhesive to your NEW battery

6. Insert the NEW battery as shown.

7. Place the shown end of the battery lid and then lower down the other end. Note that the battery pull-out tape is left under the battery lid.

8. Press and hold both of the two shown screws in.

9. Place the bottom cover as shown and press it into its place. If the bottom cover does not go in properly, loosen up the side screws a bit.

10. Hold the bottom cover in its place and tighten the torx + size 4 screws on both sides. That’s it!

(via)