How long should I wait before doing anything?

At least half an hour, and closer to an hour on the day a new iOS version comes out. A progress bar can sit completely still and then jump forward several steps at once. Keep the phone on the charger and on Wi-Fi and leave it alone.

Is it dangerous to restart during an update?

Restarting while the phone is preparing or downloading is normally harmless. What you should avoid is cutting the power while it is actually installing, meaning when the Apple logo and a progress bar are on screen. Let that finish.

Can I delete a half finished download?

Yes. Go to Settings > General > iPhone Storage, find the row with the iOS update, tap it and choose Delete Update. Then fetch it again under Software Update. None of your own content is affected by this.

Why does the phone say there is not enough space?

An iOS update needs several gigabytes free while it unpacks itself, considerably more than the file itself weighs. Free up space under iPhone Storage, for example by emptying Recently Deleted in Photos, and try again.

iPhone update stuck? How to get the install moving again

An iOS update that sits on Preparing Update, or on a bar that will not move, is nearly always down to one of three things: not enough free space, a weak Wi-Fi connection, or Apple's servers being busy on the day a new version is released. The phone itself is fine. The first thing to do is wait half an hour with the phone on the charger.

Make a backup before you move on to the bigger steps. Connect to Wi-Fi and go to Settings > your name > iCloud > iCloud Backup > Back Up Now. An update that gets interrupted the wrong way can in rare cases mean the phone has to be restored, and the backup is then the only place your photos and contacts still exist.

Try this first

Put the phone on the charger and leave it on Wi-Fi. The phone pauses on purpose when the battery is low.
Wait at least 30 minutes without touching it. The screen is allowed to go dark, that is normal.
Check that the internet works in the house. Open a web page on another phone or a computer. If that does not work either, restart the small box with lights that gives you internet: pull out the power lead, wait 30 seconds and plug it back in.
Look at the phone again after half an hour. If the bar has moved even a little, let it carry on.

If that did not help

Restart the phone. If your iPhone has no home button: press and release volume up, press and release volume down, then hold the side button until the Apple logo appears. If it has a home button, hold the side button and the home button together. This deletes nothing.
Free up space if the phone said space was missing. Settings > General > iPhone Storage. Empty Recently Deleted in Photos and remove downloaded films and podcasts. Aim for at least 10 gigabytes free.
Delete the half finished download and start over. Settings > General > iPhone Storage, find the row named iOS with a version number, tap it and choose Delete Update. Then go to Settings > General > Software Update and fetch it again.
Try a different network if yours is slow. A friend's Wi-Fi works well, as does a personal hotspot from another phone if you have mobile data to spare.
Wait until the evening or the next day if the version came out today. Many millions of phones are fetching the same file at once, and it goes faster later.
Update using a computer as a last step. Connect the phone with a cable, open Finder on a Mac or the Apple Devices app on Windows, select the phone and click Update. Choose Update, never Restore, because Restore erases the contents.

Stop and ask for help if the phone shows the Apple logo for more than an hour, shows a picture of a computer and a cable, or restarts over and over. The next step needs a computer and can erase the phone if it is done in the wrong order. Write to us and describe what is on the screen and we will go through it with you.