Samsung has today realised the latest stable version of One UI to the original Galaxy Z Flip 5G. It’s the latest stable version of the interface with some improvements. You should know though that this update doesn’t bring huge improvements.
One UI 5.1.1 for the Flip comes with improved controls for Flex Mode, as an example for the camera in the device. But this update also updates the smartphone to the August security patch. The update you should be looking for is called F707USQU6IWI1 and Samsung are sending it out in the US right now, but I think it will be available in more countries soon.