i want to update my iphone via itunes and i already downloaded the latest version of itunes but when i click update it say to me you need to install the latest version

Reboot your router and computer, temporarily disable any VPN, and try again.

See If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support. If you're trying to update using iTunes try over the air instead, or vice versa.

Note if you're getting a -4000 error when updating from iTunes you need to remove your device passcode, update, then enable again afterwards.

tt2

At a guess you are running Windows 7 or 8 which are limited to iTunes version 12.10.11.2. A more recent version of iTunes will be required to work with iOS 15, which in turn will need Windows 10 or 11 to install.

tt2