How do I install Ipad IOS 16.5.1 with less than 20% battery?

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software, as outlined in this support article --> Update iPadOS - Apple Support

What you say about not being able to use a charger until it is updated is contraindicatd by:

Update your iPhone or iPad wirelessly

1. Back up your device using iCloud or your computer.
2. 
   
3. Plug your device into power and connect to the internet with Wi-Fi.
4. Go to Settings > General, then tap Software Update.
5. 
   
6. If you see more than one software update option available, choose the one that you want to install.
7. 
   
8. Tap Install Now. If you see Download and Install instead, tap it to
9. download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.

You cannot.

You must plug it into power if its under 50% battery.

You don't need an update to charge the iPad.

Unless you are using the referenced Camera Adapter, charging should not be an issue.

If you are using an adapter or connector between the iPad and the charger remove it, and use the charger directly.

Are you experiencing difficulties with charging your iPad with a particular Power Adapter or cable type?

Recent models of iPad are supplied with (and will optimally charge from) a 20W USB-C Power Adapter.

Some issues have been reported with iPad models that have a USB-C port when charging is attempted from older USB Type-A Power Adapters.