You must uninstall the 126.96.36.199 release of X6 and reinstall the 188.8.131.52 release of X6 before installing X6 MU1. The 184.108.40.206 version is the only version available on our download site.
NOTE: if you have modified any of the user customizable files (ie tools, defaults, posts, MDs, etc) that were installed with Mastercam you will want to back them up prior to performing an uninstall. The uninstall will remove all files installed during the install of X6 Mastercam regardless of if they were modified or not. Any user created files will be left in tact after running the uninstall.