Why my iPhone don’t want to update?

If your Apple mobile device won't update, read this support article --> If your iPhone or iPad won't update - Apple Support

Another, less common reason for not being able to update is if you were using a beta version of software. Refer to How to uninstall iOS or iPadOS beta software - Apple Support

Based on the information provided, it is difficult to say.

What happens when you attempt to update?

Any error messages?

Which device?

What is the existing version of iOS?