Couple of ways. You can make s new Unibeast installer with High Sierra using Unibeast for High Sierra. Then you would install just like you did Sierra except that you don't format the hdd/SSD that Sierra is installed to. That way all of the data and progams you've added to Sierra don't get erased. The other way is to update via the Mac App store just like on a regular Mac that Apple makes. With the second approach you would be best to update Clover and your kexts first to make sure they will work.