Why does my iPhone update fail

To update over the air from the phone all of these requirements must be met:

• You need at least 6 GB of free storage
• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone 12 or 13 series with an active 5G network connection
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.

Major updates, like 14 to 15 require the most free storage because they completely replace the old version with the new so both most be on the phone during the update. “Point” releases, like 15.4 to 15.5 require less but still can take a GB or more. Patch releases (15.4 to 15.4.1 for example) can be quite small. But you should try to have at least 10% of your storage free all the time to prevent other issues and conflicts.