Moving corners is important when you want to change the room dimensions/shape.
Move the cursor on the corner you want to move, then right click the cursor over it and select command Move (always have a look at the title of the context menu you get each time you right click the mouse button, as it reveals the object/entity the Application is referring to: if you don’t find the command you are searching for, probably you have right-clicked over the wrong object. So close the context menu by clicking outside it, and try again on the correct entity, selecting one of its construction lines).
When you enter the Corner move mood, you can drag&drop that corner somewhere else.
As usual, you can use the left mouse button for the new position, or you can be more accurate by typing the distance from the first position in the dedicated fields and click Enter keyboard button. When moving the corner, you can press the O keyboard button in order to enable the orthogonal mode, being sure to move the corner just horizontally or vertically.
This article is valid up to version 2018