How do I prevent prompts to update to iOS 10 I've got an iPhone 6 that I would like to keep running iOS9 for the purpose of testing iOS applications.  Is there any way to prevent the prompts to install iOS10?
Short answer, No.
Apple really, really, really wants you to upgrade.

Longer answer...
You'd need to be able to block the domains  

appldnld.apple.com
mesu.apple.com


on every WiFi access point you meet.