Why I can't update to the latest version?

There are at least a dozen possible reasons, so it’s impossible to answer your question without a lot more information.

As a start, is your phone eligible for an update? First, 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 Northern Hemisphere Fall, 2022
• iPhone 12 or 13 series: You can update to 15.6
• Any model 6S or newer (including original SE, 2nd and 3rd editions): You can update to 15.6
• iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.5
• 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

Is your phone eligible for an update? If so, do you meet all of the requirements to 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, 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.