How can I update my iPhone without a computer?

I don't understand what you mean - what makes you think you need a computer to update your iPhone? Unless you mean something other than updating iOS. Are you having problems updating to iOS 15 (you've stated that you've an iPhone XR running iOS 14, is that correct?)? A computer is not required to update iOS, just tap 'Settings' > General > and then 'Check for Updates'.

If you're having problems updating your phone, please post back (to this thread) explaining the issue.

See if anything here helps.

Get help with over-the-air iOS updates  -

Update – Can’t or can’t restore   -*

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

Updating iOS: common issues and solutions.  -

https://www.youtube.com/watch?v=csJAEyvvruY

As long as you have access to WiFi, it's pretty easy.

Start here:

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