It is impossible to change or update your iPad’s internal hardware - as this is fixed during manufacture; what you have is fixed for the life of your device. If you purchased a WiFi Only model of iPad, you cannot enable cellular service. If instead you purchased a WiFi+Cellular model, you can simply add a physical SIM from your chosen cellular carrier, or enable service directly from the iPad:
Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support
While you cannot add Cellular capability to a WiFi Only model of iPad, you can access Cellular service using a WiFi hotspot. There are two main ways to provide a WiFi hotspot:
1) A mobile phone - that can provide a WiFi Hotspot. An iPhone is ideal, as the hotspot connection can be initiated directly from the iPad. Other brands of mobile phone will usually suffice, but the hotspot connection must be manually enabled each time that you wish to use it. It follows that the Cellular contract must allow connection via the WiFi hotspot (otherwise known as tethering).
How to set up a Personal Hotspot on your iPhone or iPad - Apple Support
2) A flexible alternative (and relatively low cost option) is to purchase a mobile broadband router - often known as MiFi - and an appropriate cellular airtime contract from your chosen Cellular provider.
A hotspot connection can also be used with WiFi+Cellular models of iPad. In this regard, both WiFi Only and WiFi+Cellular models are equal.
