If your device won't update, make sure you are following all the conditions for the method you are trying to use. If you are sure you are following directions correctly, try a different method.:
Update your iPhone or iPad - Apple Support - wirelessly*
Update iOS on iPhone - Apple Support - with a computer for an iPhone, or a related computer one for an iPad Update iPadOS - Apple Support
If your Apple mobile device won't update, read this support article for a list of possible reasons --> 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
credit: limos