You're looking to edit the wrong component, leave UVs alone unless you're going to prepare the model for texture painting, and even then, you cannot move,scale, or rotate Uvs in 3D space, it must be done via the UV Editor, hence the warning message. Faces, vertices and edges are the components you manipulate to edit the mesh, not Uvs.

The insert edge loop tool is your friend. Drop a row of edges in the height, width and length, the closer the edges are, the less its going to be rounded when you smooth.