my iphone could not be updated. An unknown error occurred

What iPhone model? what iOS version is currently running on it?

How are you trying to update it, wirelessly or via computer?

Should be the former, see this support article:--> If your iPhone, iPad or iPod touch won’t update

If you get no help, try to update it using a computer running iTunes (PC) or iTunes/Finder (Mac).

If the issue persists, contact Apple Support using this link:--> Get Support

Regards

Giulio