Can you help me update my phone please?

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 15..

Supported iPhone models - Apple Support

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

If your details are correct and you have a 6s running iOS 13, you are years behind with your updates, you may need to use a computer to do the update.

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..