LG G3 D855 only rooted, I want to unroot update and root again

I've rooted some months ago my D855, I rooted it only because I want to write on the SD and use some software that requires root. Since 1 month my G3 says to me that there is a google patch that increase the usability. I'd like to update, but when I try to do this it says to me that rooted devices cannot be updated.
Then I'd like to unroot my device, update, and then root it again, but I'd like to do this without miss my data. How can I do? Some hint?