For getting into edit mode make sure that the shape with the label that you want to edit is the only selected shape. When you move the mouse pointer into the single selected shape, the mouse pointer turns into a .By clicking button-1 on the shape or typing the first character you go into edit mode. See section 2.5.1 for the different text edit commands that exist in TCM. These apply to all document editors including all diagram editors.
Each TCM diagram and tree editor has a special node type called Comment, which solely consists of a text label and is intended to add comment to the diagram. This node type cannot be connected by any other edge, except in TGD, the generic diagram editor. But comment nodes can be selected, moved and of course edited.
Tip: If you want to edit the label of an edge, and you try to select it first with button-1 you could miss the edge and create an unwanted node. To avoid this, make the selection empty and select the edge by button-2 instead, because, when you miss then no node is created.