5S backup problem

Some time back I re-set my iPhone 5S - now when I want to backup the content iTunes says ---

"Welcome to your new iPhone" and asks me if I want to set it up as a new phone or restore from an year-old pre re-set backup. But what I only want to do is record the current state as a new backup. How can I do this ?

(Computer is a 2011 Mac mini., OS High Sierra, latest iTunes, iPhone 11.0.3 OS)