How can I update my iPhone 6 Plus?

An iPhone 6Plus cannot update past iOS 12.5.7. It is the most recent iOS version it supports.

If yours is not already running that, then go to Settings ➜ General ➜ Software Update

You might want to update your iPhone 6 Plus, but it is not possible to update to iOS 13, 14, 15, 16, 17 or 18.