How do i use cellular data to download software update

I am able to update iOS using cellular data, but automatic updates must be turned on and I have to wait for about a week after the update comes out before the option is available. I just keep checking for updates until the download is clickable, I click on it and then I am asked if I want to use cellular data and I click yes. I’m on a 6s. Everybody says it can’t be done, but if you wait for it, it can be done.

You cannot. You can only download on Wi-Fi.