No. The battery in the N8 is a sealed unit and as such any attempt to remove it will invalidate the warranty. Also, the procedure to remove the battery cannot be discussed here.
You can buy higher mAH batteries online but they will not be genuine as there is no higher mAH for genuine N8 battery. If you are technical enough to change the battery with a replacement then do so, BUT THIS MIGHT VOID YOUR WARRANTY.
This is because, Nokia recommends you only use their genuine replacement batteries, and with the N8 it should be given to a Nokia Care point to do it for you as it is not user replaceable, although some people claim to have done it.