Operating systems are closely tied to the hardware on which they are installed. Changing the operating system is typically automated through a bootable disc, but at times can require changes to the hard drive. Changing the operating system can cause loss of data or even a temporary disabling of certain hardware components.
How Operating Systems Are Changed
When installing an operating system from scratch, you'll typically need a bootable USB drive or stick. "Bootable" means that the drive or stick is capable of superseding your current operating system's boot process. Once you boot to the drive, the process is similar to the installation of any other program or app. Some installers come with a software program that will initiate the boot process from within the current operating system, but you should check the documentation for the new operating system to ensure compatibility.
Caveats to Operating System Change
Unlike an app or program, an operating system must modify the boot record of your hard drive, particularly if you're changing operating system makers, such as from Linux to Windows. This might require the hard drive to be re-formatted or re-partitioned, a process that erases all of the data on the hard drive. Multiple operating systems can be installed on a computer, with the first one installed often serving as a master operating system that manages the others. For example, Boot Camp enables the loading of Windows software alongside MacOS X. Regardless of the type of OS installation, always ensure you have backed up documents on an external hard drive before beginning any operating system change.
Upgrades versus Clean Installs
If you're keeping the same manufacturer for your operating system, you can likely upgrade your operating system like any other program. Windows and OS X allow you to run upgrade programs that will change the operating system, but leave settings and documents intact. In this instance, you don't have to "boot" to the upgrade disc. Be sure to check the compatibility of your hardware and software with the new operating system -- upgrading from Windows 7 to Windows 8.1 might be seamless, but going from Windows XP to Windows 7 will require the old operating system to be erased, a process referred to as a "clean" install.