Toggle Airplane mode ON. You can do this by swiping up (iPhone 8 or earlier) or down (iPhone X or later) to bring up Control Center.
Tap to launch Settings from your Springboard and select General from the Settings menu.
Choose iPhone Storage from the General menu then tap to select your software update from the list of items taking up space on your device. For example, "iOS 12.1".
Hit the large Delete Update button then tap Delete Update again to confirm removal of the software update from your device.
You'll be notified that the Software Update Failed. Tap Close to dismiss the alert.
You can now toggle Airplane mode OFF and install the update at a later date.
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