re: cube subsurf once then how do I shrink mesh to new object?

I have seen this done in a tutorial but unfortunately cannot find it again, they was a shortkey or option in Blender where after subsurf on a cube the mesh stays on the original cube. How do I update the mesh to the curvier object? Thanks.

on the subsurf modifier there is an apply button - hit that.

In the Modifier tab, where it says ‘Catmull-Clark’ choose ‘Simple Subdivide’.