Update to a specific version of Safari for iPadOS
The version of Safari browser is tied to the version of iPadOS running on your iPad. It's not possible to pick and install a specific version of Safari which is lower than the current latest version of iPadOS.
If you wish to use a specific version for development and testing, you can install Xcode on your Mac, which would give you access to iOS/iPadOS simulators running on macOS.
Thereafter you can pick and download a specific older version of iPadOS (such as iPadOS 13.1).
The only con is that instead of Safari running on a real iPad device, you'd be testing on a software simulator of Safari for iPad running atop a desktop/laptop computer.