How can I download a new software version

You can download the latest version supported by your iOS device, by going to Settings > General > Software Update. Make sure that you have a good internet connection and your device has at least 50% of charge.

Have a look on the following support article: Update your iPhone or iPad

If it says the iOS is up to date you may have a beta version installed.

If so, click on the link below for instructions on how to remove a beta profile -

How to uninstall iOS or iPadOS beta software - Apple Support How to uninstall iOS or iPadOS beta software - Apple Support

May I ask which device are you trying to update ?