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.
NO you can not replace the battery of N8 with the higher current rating, higher current rating meaning you are using a battery that a little bit bigger in size, so since the N8 is a sealed and no space for a bigger battery you cant use a bigger battery.
If you don't care about the warenty then you can look at replacing the battery with the unit from the 808
There is a detailed how to on all about symbian
Further discussion is not allowed here due to the voiding of the warenty this process this entales (are there still N8's with intact warenties ?)