Other contributors have said "How do you install jagex?" is the same question as "Where can you download jagex?". If you believe that these are not asking the same thing and should be answered differently, click here.
It should already be downloaded, but some computers kick it off, I suggest looking on the web, if you cannot find it, Jagex has been deleted, forever.
Actually, Runescape is just a trademark of Jagex. That's not how you download Jagex. What about FunOrb?
You can download Runescape if your web-browser does not support Runescape. It is available under the 'Downloads' section on Runescape's website.