Can I reset an iphone without accidentally deleting iCloud photos?

Our daughter gifted our Granddaughter with a new iPhone two years ago. They offered me our Granddaughter's old 6s. I want to reset the 6s but am concerned that I may disrupt or delete her photos on the iCloud account. How can I reset the 6s and leave the iCloud account intact?

I guess this is a "hand me up" instead of a hand me down. =>

Thanks,

Dan