How can I upgrade my iPhone

go to Settings > General > About.. what does it say for Software Version?

If your details are correct and you have an iPhone 6 Plus running iOS 12, you can go up to iOS 12.5.5

You cannot update beyond iOS 12.

If you need to run newer iOS, then you'll have to get a newer iPhone model

below are some details about updating:

have you read thru this support article for updating?

Update your iPhone, iPad, or iPod touch - Apple Support

need more storage for update?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

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