how do i finish updating iphone by using another device?
I updated recdently and it tells me to go to one of your other devices signed in to i/clouod to approve this iphone
To access iCloud on another iDevice, you need to Request the iCloud Desktop Site. For more info, check out this article: https://appletoolbox.com/2013/06/how-to-login-to-icloud-com-on-iphone-or-ipad-q-a/
This should help you finish up.