I want to know how to upgrade more storage in my iPad 9th gen

It is impossible to increase the amount of RAM or internal storage in your iPad - as this is fixed during manufacture; what you have is fixed for the life of your device. You can, however, subscribe to a higher tier of iCloud storage - but this is not a substitute for internal storage.

Upgrade to iCloud+ - Apple Support

If using iCloud Photos, this is a effectively a synchronisation service; your iPad and iCloud both have copy of your photos - although it is possible to configure your iPad to keep a low-resolution version of the full-resolution images that are kept in iCloud:

Settings > Photos > iCloud Photos - configure settings as required

Note that deleting photos from your iPad will also delete the corresponding image from iCloud Photos. If you wish to keep photos stored in iCloud alone, you'll need to create a separate folder in iCloud that is independent of iCloud Photos - and manually copy/move your photos into this folder before deleting photos from your iPad; some third party File Manager Apps can be helpful with this process.

If your iPad internal storage is full, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

“Offloading” Apps can be a helpful method of creating temporary space for a software update.

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

Additional information about creating sufficient space for installation of a software update can be found here:

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

External storage options can often help with managing limited internal storage. Depending upon your iPad model, the iPad will either have a Lightning or USB-C port - and may therefore require either an Adapter or USB hub to connect the USB Storage device. If your iPad has a Lightning port, you’ll need an Apple Lightning to USB3 Camera Adapter:

• Lightning to USB 3 Camera Adapter

https://store.apple.com/uk/xc/product/MK0W2ZM/A

You cannot put more storage in an Apple device, what you get when you buy it is what you get for the life of the device. This is why when you buy it, you need to make sure that you get enough storage to do what you want to do. If you are full from pictures and videos, you can always connect your iPad to your computer and transfer the images and videos to the computer so that you can remove them from the iPad and get storage back. If you are having your iPad sync to iCloud and you delete them on your iPad they will be removed from your iCloud as well.