How can I update my iPhone without WiFi

You can update using a computer running iTunes or a Mac via the Finder if it is running Catalina or Big Sur.

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

If you don’t have a computer or access to one, you will have to find a WiFi network where you can update.

You would then need to update through iTunes on your computer. See the instructions in the related article link at the top of the page.