How do I download software update over cellular data

You don't.

iOS updates require Wifi since they are very large, and can easily consume all the cellular data in a plan and cause expensive overages to people without a plan with unlimited data. They also need a stable connection, which cellular does not have unless you are staying static in a single location.

Connect to stable Wifi network to download the update.

Settings>general>reset>reset network settings to fix the greyed out wifi.