Well you need the DVD of Windows 7 to install, when you stick the DVD in it will ask if you want a fresh install or upgrade.
However, depending on your system I would upgrade to Windows 10, Windows 7, although a good system, is no longer supported by Microsoft (security updates).
Do you already have a Windows 7 DVD?
Is this a laptop or PC?
How much memory does it have?
If you list your specs we can advise:
Memory
Hard Drive Size
Also run this:
https://www.microsoft.com/en-us/down...ils.aspx?id=20