Help me to update my iPhone to 15 iOS

Go to Settings/General/Software Update and follow the instructions. 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)
• 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.