How do I delete this or any update?

Unless you have installed a beta version of iOS, you cannot delete your current version to use an older one. To stop future updates, turn off Automatic Updates and simply do not install anything else. To do so, tap Settings > General > Software Update > Automatic Updates: Set all of the switches to OFF. This will require you to update your iPhone manually when you choose to do so.

If you're talking about an update that has been downloaded but not installed, see here:

1. 1. Go to Settings > General > [Device name] Storage.
   2. Find the update in the list of apps.
   3. Tap the update, then tap Delete Update.

If you're asking about an update that has been installed, you can't delete it. Downgrading is not supported.