Shading

Reference

Mode

Edit Mode

Menu

Mesh ‣ Shading

Alternatively, you can choose which faces to smooth by entering Edit Mode, then selecting some faces and picking Shade Smooth from the Face Menu.

When the mesh is in Edit Mode, only the selected faces will receive the 《smoothing》 attribute. You can set faces as flat (removing the 《smoothing》 attribute) in the same way by selecting edges and picking the Shade Flat from the Face Menu.