Why it works when the phone alone won't
An over-the-air update has to download the new system onto the iPhone and unpack it there alongside the version already running, which is why it wants roughly twice the download size in free space. Updating from a computer moves both jobs off the device.
On a 64 GB iPhone with a full photo library, that difference routinely decides whether an update is possible at all. It also avoids the corrupted-download failures that come from unstable Wi-Fi, because the computer handles the transfer.
On a Mac
Connect the iPhone with a cable and open Finder. The device appears in the sidebar under Locations; select it, and if this is the first connection you will need to tap Trust on the phone and enter its passcode. On the General tab, click Check for Update, then Download and Update.
Leave the cable connected until the phone has restarted and reached the Home screen. Disconnecting during the install is the one thing that turns a routine update into a recovery-mode restore.
On Windows
Install the Apple Devices app from the Microsoft Store — it replaced iTunes for this purpose on modern Windows. Connect the iPhone, trust the computer when prompted, select the device and choose Check for Update.
If you are on an older Windows setup still using iTunes, the same options live under the device Summary tab. Either works; the Apple Devices app is the current one.
Update, not restore
The two buttons sit next to each other and do very different things. Update installs the new version and keeps your data. Restore erases the iPhone and installs a clean copy. Read the button before clicking it.
If you deliberately want the clean install — because something is genuinely broken rather than merely out of date — make a backup first and see our restore guide.
If the computer cannot see the phone
Try a different cable first: a cable that charges reliably may not carry data, and this is the single most common cause. Then a different port, then restarting both devices. On a Mac, check that you tapped Trust on the phone.
If the iPhone will not boot normally at all, the computer can still reach it in recovery mode, which is the route back from a failed update — our recovery mode guide covers it.
Frequently Asked Questions
Can I update my iPhone without Wi-Fi using a computer?
Yes. The computer downloads the update over its own connection and installs it over the cable, using no cellular data on the phone.
Does updating from a computer erase my iPhone?
No, provided you click Update rather than Restore. Restore erases the device; Update keeps your data.
Do I still need iTunes on Windows?
No. The Apple Devices app from the Microsoft Store replaced it for this purpose, though older iTunes installs still work.
Why won't my computer see my iPhone?
Usually the cable. Try a known-good data cable, a different port, and make sure you tapped Trust on the phone.