How can I upgrade to iOS 14

If your signature is correct and you have iPhone 7, you can't upgrade to iOS 14 because it is no longer available, but you CAN upgrade to iOS 15.7. Anything which requires iOS 14 will work just fine under iOS 15.7.

See this --> Update your iPhone or iPad - Apple Support

Note: If you haven not updated your phone in a long time, you may need to use your computer to do the update.

You can't, if you have a 7 Plus you can update to iOS 15.7