Install Snow Leopard right on top of your existing Leopard install.
Upgrade: This is the easiest option - simply upgrade. When it comes to upgrading to Snow Leopard, you’ve basically got two options: upgrade Leopard or wipe the old OS and start fresh. How do I upgrade from Leopard to Snow Leopard? You can upgrade directly from Snow Leopard to any newer version of OS X, including the current El Capitan, and the Mac App Store is the main delivery method for those upgrades.
… Once you install the Snow Leopard, you can use the Mac App Store to update to any of the later versions of OS X, as well as purchase and install many apps for the Mac. OS X Snow Leopard (10.6) is considered the last version of the Mac operating system that was predominantly designed without influences from iOS devices.