Cancelling a download
While an update is downloading, Settings → General → Software Update shows its progress and you can simply leave the screen; the download pauses rather than continuing in the background indefinitely. To remove it entirely, go to Settings → General → iPhone Storage, find the iOS update in the app list, tap it and choose Delete Update.
That reclaims the space and clears the badge. Nothing is harmed — the update is just a file at this stage.
Do not interrupt an install
Once the phone has restarted and shows the Apple logo with a progress bar, it is writing the new system. Forcing a restart here can leave the device unable to boot, and the way back is a recovery-mode restore that erases it.
If you started an install you did not mean to, the safe course is to let it finish and then decide what to do. A completed update you did not want is a minor annoyance; an interrupted one is a bad afternoon.
Stopping it from downloading again
Deleting the update is not enough on its own. If Settings → General → Software Update → Automatic Updates has Download iOS Updates switched on, iOS will re-fetch it within hours and the badge will reappear.
Turn that off as well. Our automatic updates guide explains the two switches and which one actually stops the behaviour you are seeing.
Cancelling a scheduled overnight install
When iOS has staged an update and scheduled it, Software Update shows the planned time with an option to install tonight or later. Choosing a later option defers it; turning off Install iOS Updates cancels the schedule outright.
This is the setting behind most cases of waking up to an unexpected new version.
If you already updated and want to go back
Going backwards is much harder than not updating. It requires a recovery-mode restore from a version Apple still signs, which erases the device, and a backup made before the update to restore from — a backup made afterwards cannot be restored onto the older version.
Our IPSW guide covers what is still signed and how the restore works. In most cases the honest answer is that staying on the new version is the better option.
Frequently Asked Questions
Can I stop an iOS update that is downloading?
Yes. Delete it in Settings → General → iPhone Storage, and turn off Download iOS Updates so it does not return.
Can I cancel an update that has started installing?
No, not safely. Interrupting the install can leave the iPhone unable to boot, requiring a restore that erases it. Let it finish.
Why does the update keep downloading again?
Automatic Updates has Download iOS Updates switched on. Turn it off as well as deleting the staged update.
Can I go back after updating?
Only through a recovery-mode restore to a version Apple still signs, and only with your data if you made a backup before updating.