Typically, when you install a new version of the Second Life Viewer, you can simply install over your existing version. However, if you are experiencing problems, one of the first things to do is completely uninstall your current version and reinstall the same or a newer version. Sometimes this is referred to as a clean install.
A clean install may help if there is an unusual issue affecting the Viewer user interface (UI), or if a Resident is having difficulty loading UI elements such as windows or menus. It can also be done to restore lost settings or return things to a default setup, if somehow the settings in the Viewer have changed or become unusuable. It doesn't usually hurt and can sometimes fix things that otherwise defy explanation.
It's important to clear out temporary Second Life files (such as the cache) when reinstalling.
Note: Uninstalling deletes your chat and IM logs. If you want to keep them, save the files before you uninstall. See Chat and IM logs for more information.
The procedure to use depends on your operating system: