How to update my iPhone?

Go to Settings > General > About

What does it say for iPhone Model and Software Version?

You need an iPhone 6s/6s Plus or newer in order to run iOS 14/15..

Supported iPhone models - Apple Support

If your iPhone is an iPhone 6 , then you cannot upgrade beyond iOS 12

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..

It depends on your iPhone model. Take a look at this compatibility chart: