Choosing between an upgrade and a clean install

As the name indicates, whether you are installing Windows 10 on a new hard disk or on an existing disk, the clean install (see Figure 1.1) formats the new disk so it can install the new OS, or reformats the existing disk by erasing the existing OS, users' data, and apps to install the new OS. To summarize, a clean install (re)builds the OS and its utilities entirely from scratch, thus overwriting all the previous content on a disk:

Unlike clean install, upgrade (as in Figure 1.2) replaces your existing OS with a new one. That means that you retain your files and settings. It is often called an in-place upgrade because it takes place on a machine with an already installed OS. Prior to running an upgrade, it is recommended that you make a backup of Windows system states, files, and folders: