how to download software update by mobile data

Follow the steps in the Support document I linked to above. Specifically the section titled: Update your device using iTunes:

1. Install the latest version of iTunes on your computer.
2. Connect your device to your computer.
3. Open iTunes and select your device.
4. Click Summary, then click Check for Update.
5. Click Download and Update.
6. If asked, enter your passcode. If you don't know your passcode, learn what to do.

You can't. Cellular / mobile downloads are limited to 100MB by the carriers. Software updates generally exceed this limitation.

You can either use Wifi, or download the update on a computer running iTunes and update that way.

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

Not much more to explain.

There's a limit to how large a download can be on cellular data. When I answered this back in January, then limit was 100MB. It's since been increased to 150MB. Its a cellular carrier limitation Apple chooses to follow and enforce.

Why the enforce it, like because many people do not have unlimited data and that would make downloads over cellular cause many financial issues to these people if they go over their limit and get overcharged for downloading 1 or 2GB of update over cellular. Going over your cellular data limit can be very very expensive depending on your carrier.

With Wifi being as accessible as it is today, it should be a simple thing to find a Wifi network to connect to to download the update, or use a computer.

You cannot update over cellular. If this is a feature you require, you can send Apple feedback to request it, and maybe they'll consider making it possible in a future iOS update. Feedback - iPhone - Apple

can you explain more accurately because what can not use cellular mobile, and is there no other way how i can use cellular data.