unable to install update

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
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone

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

Above related article link has details.

Battery charged, Wi-Fi connected and not cellular, no add-on VPN apps in use, and sufficient free space for the update are required.

If you lack free space for the download, use a Mac or Windows system for the update.

If that’s not working and you’re receiving errors or diagnostics, please post the messages.

If you have an iPhone 6 or earlier, no updates past iOS 12 are available, nor will there be.