How can I upgrade my iPhone
how can I upgrade my iPhone software
iPhone 6 Plus, iOS 12
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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
video..