On rare occasions users may find it necessary to downgrade to an earlier version of Lightroom.
Windows: To uninstall the current version of Lightroom for Windows - open Windows Control Panel and choose Add/Remove Programs.
Mac: To uninstall the current version - open OSX Library folder (i.e. main Library not the Users Library), navigate to folder named Receipts, select the package named Adobe Photoshop Lightroom.pkg (or similar), drag to trash. Open Applications folder, grab Lightroom, drag to trash.