I already downloaded software but I cannot install for update version 15 since I said error occurred during installation.

Restart your iPhone and make sure you have enough storage to install the update.

If this doesn’t work, then go into Settings > General > iPhone Storage > iOS 15 and delete the update, then go back and re-install the update.

Make sure you aren’t using any VPNs or proxies.

If all of the above doesn’t work, you will need to update using a computer.

Learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.

Verificar o cabo poder ser mal contato ou sujeita. O meu aconteceu tbm