upgrade from macOS10.6.8 to Sierra, how??

I have a Mac Book Pro from 2011, with mac OSX10.6.8 as operating system. I cannot upgrade even to El Capitan!, PLEASE, how can I do it?

Tengo un MacBook Pro del 2011, con mac OSX 10.6.8 . Por favor, cómo actualizo a Sierra desde un sistema operativo tan antiguo?? Favor ayuda.