After you run the update from the computer where 3CX is installed, go check the version number of the 3CXPhoneSystem.exe. You will see that it shows version 5.1.4287.0 although the server status webpage will say otherwise. (To check that out use the file explorer, navigate to the directory where 3CX is installed , go under "bin" and check the properties of the file 3CXPhoneSystem.exe). Since it's just an update, apparently the setup page is not properly adjusted, but will have the latest.
Make sure that you run the update program on the computer where 3CX is installed. You don't do it from the webpage.
Halea