calendar only goes back one year

Greetings, flxible.
Thanks for using Apple Support Communities. From reviewing your post it appears that the Calendar app on your iPhone 6 Plus on iOS 10 isn't showing you more than one year's history. I'll be glad to help.
Before you begin, I recommend that you create a back up of your data. Take a look at this website for more information:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Force an app to close on your iPhone, iPad, or iPod touch -- First, use these instructions to force close the Calendar app if it's open, then Restart your iPhone, iPad, or iPod touch -- Never underestimate the power of a good restart.
Remove built-in Apple apps from the Home screen on your iOS device with iOS 10 - Apple Support

Remove a built-in app from your Home screen

1. Touch and hold the app lightly until it jiggles. If the app doesn't jiggle, make sure that you're not pressing too hard.
2. Tap the "X" on the app, then tap Remove.
3. Press the Home button to finish.

Next, tap App Store > Updates > Purchased, locate the Calendar app and reinstall it. You'll need to re-enable your accounts through Settings > Calendar > Accounts by tapping each account and turning on "Calendar".

Have a great day!