On Windows, download the plugin file from
here:
Then to update on to beta on Windows
Close Lightroom (important)
Unzip the download
Delete (or rename and move) the existing NegativeLabPro3.lrplugin file from its location
C: \ Users \ [username] \ AppData \ Roaming \ Adobe \ Lightroom \ Modules \ NegativeLabPro3.lrplugin
Put the unzipped NegativeLabPro3.lrplugin file in that folder where the previous plugin used to be
Restart Lightroom
Go to “File > Plugin Manager”. Add Negative lab Pro. In the “Status” section it should now say v3.1