How do i upgrade my Ipad to a specific software without a computer?
It is only possible to upgrade to a specific version if your device doesn’t support the latest version or if Apple signs the update, so you can update using iTunes.

All devices that support iOS 11 support iOS 12 so that is not an option.

Apple is no longer signing iOS 11 and hence it isn’t possible to update to it. If you really need iOS 11 you can buy a used/new device online that is running iOS 11, but that is your only option.