Update

Software Update may incorrectly say that an unit is up to date if there's a beta profile installed or it can't properly communicate with Apple's servers.

Unenroll Your Devices [all kinds] to Stop Receiving Public Betas - https://beta.apple.com/sp/en/betaprogram/unenroll - select the correct OS version.

Click on a link below for full instructions about how to update with wi-fi or 5G, or using a computer if you have storage or connectivity issues.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> https://support.apple.com/HT204204

Update your iPhone, iPad, or iPod touch with the Finder [Catalina or later] --> https://support.apple.com/HT212185

Update your iPhone, iPad, or iPod touch with iTunes [Mac Mojave or earlier; Windows] --> https://support.apple.com/HT212186

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

Check for a beta profile. If found, delete the profile and attempt to update again.