First make sure that you’re in wireframe mode and the interior edges are displayed. You can find this option under “Display” in the main menu.
Then select the appropriate face(s) by clicking on the interior edges. Selected faces will be drawn yellow. (Tip: By keeping the Ctrl key pressed while selecting a face all faces attached to it belonging to the same layer will be selected in one go)
Finally select the layer in the drop down box in the layer toolbar where you want to move the faces to.