Turn on cellular downloads
Open Settings → General → Software Update → Automatic Updates and look for the option covering cellular downloads; on iPhones that support it, Settings → Cellular → Cellular Data Options also carries a Data Mode setting that must not be on Low Data Mode. With those set, Software Update will offer to download over cellular when Wi-Fi is unavailable.
Apple gates this deliberately. Depending on your carrier, your plan and the size of the release, the option may not appear at all — and on a 5G iPhone it is more likely to be offered than on an older LTE model.
Know what it will cost you
A major iOS release is several gigabytes. On a capped plan that can be most of a month's allowance in one download, and on a roaming plan it can be genuinely expensive. Software Update shows the download size before you commit — read it before tapping.
A minor point release is far smaller, often only a few hundred megabytes, and is much more reasonable over cellular. If you are trying to get a security patch rather than a whole new version, check which one you are actually being offered in Settings → General → Software Update.
Personal Hotspot from another phone
iOS treats a Personal Hotspot as Wi-Fi, so an iPhone connected to another phone's hotspot will download an update without any cellular setting involved. The data still comes out of the host phone's plan, but the restriction disappears.
This is the practical answer when your own iPhone will not offer the cellular option. Connect to the hotspot in Settings → Wi-Fi, then run Software Update normally.
The computer route uses no mobile data at all
If you have access to a computer with any internet connection, updating through Finder on a Mac or the Apple Devices app on Windows downloads the update to the computer rather than the phone. No cellular data is used, and the iPhone needs much less free space because the computer does the unpacking.
This is the most reliable option on a device that is short of both space and bandwidth. Our computer update guide covers the steps.
Keep the connection stable
A partially downloaded update over a weak cellular signal is the most common cause of an install that fails at the same point every time. If the download stalls or the install refuses to proceed, delete the update in Settings → General → iPhone Storage and start again on a stronger connection.
Our guide to updates that will not install covers the other causes in the order worth checking.
Frequently Asked Questions
Can I update my iPhone using mobile data?
Yes, on supported models and plans. Enable cellular downloads under Software Update, and make sure Low Data Mode is off. Apple does not offer the option on every device or carrier.
How much data does an iOS update use?
A major release is several gigabytes; a point release is often a few hundred megabytes. Software Update shows the exact size before you start.
Does a Personal Hotspot count as Wi-Fi?
Yes. iOS treats a hotspot connection as Wi-Fi, so an update will download over it without any cellular setting being involved.
Can I update with no internet on the phone at all?
Yes, through a computer. Finder or the Apple Devices app downloads the update itself and installs it over the cable.