how to find the serial number of an iphone 6 that i recently took over. where is it on the phone?

The serial number can be found under settings>general>about. You can hold your finger over it until "copy" appears and then copy and paste it into notes in your desired size. Even if only part of the number is visible in the about menu the entire serial number will copy. The serial number can also be found on the original box and in iTunes. -AJ

Hi. Check this article: Find the serial number and other information for your iPhone, iPad, and iPod touch - Apple Support