Q: How to upgrade from 10.6.8 to Snow Leopard on older Imac

Your iMac would have to be a Mid 2007 or newer to upgrade to El Capitan > Upgrade to OS X El Capitan - Apple Support

If your iMac meets El Capitan requirements, El Capitan v10.11 is available for download for free at the App Store.

If you do this, make sure to backup all your important data first.

If you use Rosetta >  Upgrading to 10.7 and above, don't forget Rosetta!

What Mac model do you have exactly?

What do you wish to update to?

If it supports El Capitan you can update through the Mac App Store.

http://www.apple.com/osx/how-to-upgrade/

If it does not support el Capitan,  you may be able to update to Lion by buying a download code for it here:

http://www.apple.com/shop/product/D6106Z/A/os-x-lion

and downloading it from the Mac App Store as well.

from 10.6.8 to Snow Leopard

FYI, 10.6.8 = Snow Leopard. The next OS version is Lion and requires more RAM than Snow Leopard. If your Mac is quite a bit older, I'd consider staying with what you have and save for a newer model. The new OS versions run best with lots of RAM (8 - 16 GB) and newer/faster processors.