Upgrading a Revit installation involves moving from a previous release of the software to the newest release.
If desired, you can install the newest release of Revit without uninstalling the previous release. The new release and the previous release can coexist, side-by-side, without issues.
When you open an existing Revit project using the new software release, Revit automatically upgrades the project as needed to work with the new release.
Revit is not backward-compatible. After you upgrade a project to the new release, you can no longer use that project with an earlier release of the software. If you want to continue to use the previous release of the software with a particular Revit project, you must first open that software release (for example, open Revit 2013), and then open the project from within Revit. If you just try to open the project from Windows Explorer, for example, it may open using the new release of Revit.
To upgrade your stand-alone Revit installation, install the new release of Revit on your system, using the instructions provided. Uninstall the previous release of Revit when you are ready to do so.