I want to update my iPhone to the latest iOS

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. 

Remove any add-on VPN client apps, remove any beta profile, ensure sufficient storage is available, force a restart, connect to Wi-Fi, and try again:

Update your iPhone or iPad - Apple Support

How to uninstall iOS or iPadOS beta software - Apple Support

Force restart iPhone - Apple Support

If that fails, connect to a Mac or Windows PC, and update that way. That is described in the how-to-update article.

If you have tried that and failed, please provide a few more details about what you’ve tried, and what error messages or diagnostics were shown.