my problem is I could not update my ipad

I’m not sure what iPad you have, but according to your profile it looks like you have the iPad Air 2. If that is the one you have then you should be able to update it as that model is compatible with the newest iPadOS 13, however, if that isn’t the model that you have then you probably have a model that is not compatible with that version. If you are getting a message “iPad is up to date” then it’s because your device is too old and is not able to run higher than the version it has installed currently as the hardware just cannot handle a newer version.

Your iPad Air 2 is too far behind in iOS upgrades/updates.

If you have access to a full computer, like a Mac or Windows PC, try either using the Finder on a Mac that has the newest macOS Catalina installed OR with an older Mac with an older Mac OS, download and install the latest iTunes version ( or latest iTunes for Windows PC ), make sure the Mac or Windows PC is connected to the Internet, connect your iPad by the USB charge cable to the computer and let the iTunes application upgrade/update that 2013 iPad Air model, using the iTunes Software Update feature, to its current, maximum iOS of iPadOS of 13.5.

Good Luck to You!