How do you update an ipad 2?

An iPad 2 cannot be updated beyond iOS 9.3.5 (or 9.3.6 if it is a cellular model).

If yours is running something earlier than that, see: Update your iPhone, iPad, or iPod touch - Apple Support

I'm not sure whether it is still supported but if it is, it would be done by using iTunes. See the following from the previously supplied link:

Update your device using your computer

If you can’t update wirelessly on your device, you can update manually using a computer that you trust. If your computer is using Personal Hotspot on the device that you’re updating, connect your computer to a different Wi-Fi or Ethernet network before you update.

1. On a Mac with macOS Catalina 10.15, open Finder. On a mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.

see the link for the remaining steps