Should I upgrade or choose a custom installation?
Choose to upgrade to install a new version of Windows in place of the current one, but keep your current files, settings, and programs. 
Choose Custom to install a clean copy of Windows on a specific drive or partition (this deletes the current information on the drive or partition you select); to format, extend, create, or delete partitions; if your computer does not have an operating system; to set up a multiboot system on your computer; or to delete your current files and settings and start over.