To downgrade the Galaxy Client from preview (beta) version to public version reinstalling the Galaxy Client is required.
On Windows you can access the Programs and Features panels and remove the Galaxy Client from there.
Manually removing the config.json might be required:
You can access the directory using the Run... window (WinKey+R key combination) and entering:
On Mac you will have to go to the Applications directory and move the GalaxyClient.app to trash, you will have to also remove the contents of this directory:
You can open it using the "Go to Folder" command accessible with the Command+Shift+G key combination or selecting Go > Go to Folder from the Finder menu.
You will also need to remove this file:
Now please download the latest installer available on our website:
After the installation completes start the Galaxy Client. Importing games will be required, you can do this by clicking on the GOG icon in the top left corner and selecting "Scan and import folders".