This is something they haven't bothered to fix or care to address. I've clicked "remind me later" at least 120 times so far. I've manually deleted the 1.3GB update file at least 50 times now. Here's the solution.
I am on iOS 9.3.5 iPhone 6 and will not plan on updating ever.
Here is the solution. (click the link)
Make sure you go to this url when you are on your iPhone in safari. Make sure you:
- delete the iOS file SETTINGS -> MANAGE STORAGE (if apple downloaded it to your phone in the background)
- turn off wifi, temporarily be on cellular
- click on the link on your iphone (using safari)
Read the directions and the explanation on the website. It is a godsend to me on my iPhone 6.
Since following the instructions, I haven't had a single iOS 1.3GB download in the background OR a notification to update. No issues since.