Why my device not taking update?

If your device won't update, make sure you are following all the conditions for the method you are trying to use:

Update your iPhone or iPad - Apple Support - wirelessly

Update iOS on iPhone - Apple Support - with a computer

If your Apple mobile device won't update, read this support article for a list of things to try --> 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