Want to update from OSX 10.6.8 to OSX 10.9
Mac OS X Lion - 10.7 was (is) a paid upgrade. From there, you can then ride the free major upgrade train as far as your hardware allows. 


Upgrade Macbook Pro from 10.6.8 to 10.8


I’ve heard of people jumping to ElCapitan, but never experienced it since I have lion digitally as well as on a USB key so I just rolled that path in the past. 


Upgrade iMac12,2 from Mac OS X 10.6.8


You may run into issues, so feel free to ask a specific question if you run into a specific block knowing you can’t just upgrade in one step.