iPhone 'Software Update Failed' — How to Fix It

What it means

'Software Update Failed — An error occurred downloading iOS' or 'Unable to Install Update — An error occurred installing iOS' means iOS tried to download or install a new version and couldn't finish.
The usual culprits are not enough free storage to unpack the update, a Wi-Fi connection that dropped during the download, the iPhone being on a tight battery, or an Apple server issue right after a new iOS goes live.
A failed update leaves the iPhone on its previous version still working — you just retry.

Affected Models

All iPhone models when installing iOS or security updates
Same on iPad, iPod Touch, and Apple Watch (paired iPhone)
More common on 64 GB and 128 GB iPhones running low on free space

Common Causes

Not enough free storage for the update (often needs 5 GB or more during install)
Wi-Fi connection dropped partway through the download
iPhone wasn't on the charger and the battery ran low during install
Apple's servers overloaded right after a new iOS release
Partial / corrupted update file from a previous half-finished attempt
iPhone's clock is wrong
VPN or custom DNS blocking Apple's update servers
Beta profile installed when you're trying to install a non-beta version

How to Fix It

Restart, then retry.

Press and hold the side button + a volume button, slide to power off, wait, hold the side button to turn back on.
Settings > General > Software Update > Update Now.
Many one-off 'Software Update Failed' messages clear after a clean reboot.
Delete the partial update file.

Settings > General > iPhone Storage — scroll down the list, find the iOS update entry (it shows the version number and size), tap it, then Delete Update.
Back in Software Update, tap Download and Install again to grab a fresh copy.
This fixes most 'Unable to install' errors caused by a corrupted half-download.
Free up storage.

iOS updates need several gigabytes of working space, not just the update's listed size.
Settings > General > iPhone Storage — tap big apps and use Offload App, delete photos already in iCloud, clear Recently Deleted in Photos.
Aim for at least 5 GB free before retrying.
Use a strong Wi-Fi and stay on the charger.

Plug the iPhone into power, connect to a known-good Wi-Fi (not a busy hotel/cafe network).
An Ethernet-to-Wi-Fi adapter at home gives the most reliable download.
Once the install starts, leave it alone until it finishes — interrupting is the number-one cause of this error.
Try later if Apple is busy.

Right after a new iOS release, Apple's servers are slammed and downloads fail mid-stream.
If everything else looks fine, wait three or four hours and try again — late at night your local time often works.
Check apple.com/support/systemstatus to see if iOS Software Update is amber/red.
Update from a computer (iTunes / Finder).

Plug the iPhone into a Mac (Finder) or Windows PC (Apple Devices app / iTunes) and choose Check for Update.
The computer downloads the full update file directly and pushes it to the phone, bypassing Wi-Fi reliability issues.
This is the cleanest fix for iPhones that fail Software Update over the air repeatedly.
Fix the clock, remove beta profiles, then try once more.

Settings > General > Date & Time > Set Automatically on.
If you installed an iOS beta profile in the past, Settings > General > VPN & Device Management > remove the iOS Beta profile (if you're not running a beta).
If updates fail on a fully time-correct iPhone with plenty of storage, a clean Wi-Fi, and no profile in the way, contact Apple Support — they may need to manually push the update.

Frequently Asked Questions

Did the failed update brick my iPhone?

Almost never.
A failed iOS update leaves the iPhone on the previous version, still working — you just retry the download.
The genuine risk is pulling the power or letting the battery die partway through the install, so once an update starts, leave the iPhone plugged in and don't touch it until it finishes and reboots itself.
If your iPhone did get stuck mid-install and now shows the cable / connect-to-iTunes screen on every boot, plug it into a computer and use Recovery Mode to update or restore — Apple's site has model-specific button combinations.
That recovers nearly every 'bricked' update case without data loss as long as iTunes / Finder can update rather than fully restore.