how do i delete my purchased app history off my iphone 6 plus

You can never delete anything from your purchased history. It will forever be tied to your Apple ID. You can hide your purchases using iTunes on a computer and that the best you can do,

Hide and unhide iTunes Store, App Store, or iBooks Store purchases - Apple Support

You can't, it is an integral (and legally mandated) feature of your AppleID (not the phone). You can hide them, however, and hope your fiancee/parent/whatever is not too iOS savvy: Hide and unhide iTunes Store, App Store, or iBooks Store purchases - Apple Support