How can I upgrade my iOS on my iPhone 6s

 Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

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 Model is really 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..

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