How can i update iOS 13.3 to my iPhone 6s

Unless you are currently using IOS 13, if you update your phone, it will update to IOS 15.

See the articles below.

Update your iOS device.  -

Get help with over-the-air iOS updates  -

Update – Can’t or can’t restore   -

Have you tried using iTunes on a computer or Finder if running Catalina or later to do the update?

Update software on iOS devices in iTunes on Mac. -

Phone, iPad, or iPod touch - Update with Finder.  *

iPhone, iPad, or iPod touch - Update with iTunes.  *

If you see an error when you update or restore your iPhone, iPad, or iPod    *

Updating iOS: common issues and solutions.  -

ASC YouTube for  how to update your iPhone, iPad, or iPod touch to the latest version of iOS or iPadOS —wirelessly or using your computer.