I can’t remember my passcode

Passcode or password? What software upgrade passcode?

See the Apple Support article(s) below.

Passcode - If you forgot the passcode for your iPhone or your device is disabled.      

If you forgot the Apple ID password, enter the Apple (iCloud) ID, and click Forgot Password. See if you can get a Rescue or Notification  email option if necessary.

Apple ID - Manage            

Apple ID – Forgot Password            

An iPhone 6 Plus can't upgrade the IOS past IOS 12.

See the Apple Support article(s) below. 

iOS 14 and iPadOS 14 Compatible Devices

iOS 13 and iPadOS are compatible with these devices - Apple Support.