How do I update my ipad ios 9 to ios 13?

You don't. If your iPad is stuck on iOS 9, it means you own an iPad 2 or 3, which is now close to a decade old and is simply not capable of running iOS 13 at all. If you need the newest iOS, you need a newer iPad capable of running it.

You should be able to update to iOS 9.3.5. If you can't do that over the air, you can connect the iPad to your home computer and do the update using iTunes. This link explains how to do so --> Update your iPhone, iPad, or iPod touch - Apple Support

Then your iPad is already running its maximum iOS version.

Then as MichelPM suggests, your software is as updated as it can be.