Check compatibility:
Go to Settings/General/About and note the Model Name
- NOTE: iOS 16 is not available for any models except to iOS developers and beta participants. General release will be in Fall, 2022
- iPhone 12 or 13 series: You can update to 15.6.1
- Any model 6S or newer (including original SE, 2nd and 3rd editions): You can update to 15.6.1
- iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.6
- iPhone 5: You can update only to 10.3.4
- iPhone 5C: You can update only to 10.3.3
- iPhone 4S: You can update only to 9.3.6
- iPhone 4: You can update only to 7.1.2
If your phone is eligible for an update→Update your iPhone, iPad, or iPod touch - Apple Support
And make sure you meet all of the requirements for an over the air update:
- 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 or an SE 3rd edition 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.