How to updates iPhone 6 Plus software

Hi Mariamu93

Welcome to the Apple Community

Seems iPhone 6 plus can be updated to iOS 12.5.6 and no further.

It is always wise to back up everything on your iPhone before updating.

Turn off VPN if installed.

You need a good strong wi-fi connection,

or

connecting to a Mac or PC for the update is a good idea.

How to: Update your iPhone, iPad, or iPod touch - Apple Support

Trouble-shooting: Updating iOS: common issues and solutions - Apple Community

Support if needed:

You could reach out to the iPhone Support Team:

There is a "Get Support" link at the bottom of their website (link below(

iPhone - Official Apple Support

All the best :-)