I want to upgrade my iPhone to iOS 15

Your iPhone 6 can only update as far as iOS 12.5.7

Here are two different ways to ensure the highest iOS that your device will support is installed on your device.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Hello ~ You cannot update your iPhone higher than iOS 12.5.7 either and you need a newer iPhone if you need a higher version of iOS. If you want to shop for a new iPhone click below…

iPhone - Apple Official Site

If you chose to purchase a second hand iPhone please take a moment to read these helpful links below by clicking on them to help you make an informed decision…

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~