I can’t update my phone for some reason

more details might be helpful.. such as, an error you see?

Or what your iPhone model is and the version of iOS you're trying to update to..

If you have an iPhone 6/6 Plus, it is not compatible with iOS newer than 12

have you read thru this support article for updating?

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

need more storage for update?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

video..