You cannot do that unless you have the original installer disc that came with the computer or if you have purchased the retail Snow Leopard DVD. You can contact Apple Support about purchasing replacement discs for the computer provided Apple still has copies.
Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
To contact product and tech support: Contacting Apple for support and service - this includes
international calling numbers..
For Mac App Store: Apple - Support - Mac App Store.
For iTunes: Apple - Support - iTunes.
Or your can purchase the Snow Leopard DVD.
Upgrading to Snow Leopard
You can purchase Snow Leopard through the Apple Store: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99 plus tax. You will be sent physical media by mail after placing your order.
After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store. Access to the App Store enables you to download Mavericks if your computer meets the requirements.
Snow Leopard General Requirements
1. Mac computer with an Intel processor
2. 1GB of memory
3. 5GB of available disk space
4. DVD drive for installation
5. Some features require a compatible Internet service provider;
fees may apply.
6. Some features require Apple’s iCloud services; fees and
terms apply.
