To edit the room first you need select it, in appeared "Modify" window you have two modes to edit rooms: moving walls (Move Wall) or moving corners (Move Corner). Click on the button with any mode.

How to work with corners in point mode (blue points)
To add: Double-click on the corner and it will chamfer this corner.
To Delete: When moving a corner, if you combine two corners, the corner that you tapped to move to the other corner will be automatically deleted.

How to work in wall mode (green points)

To add: Double click on the green point and the wall will split into two walls
To delete: When you move a wall, if the length of the wall becomes equal to zero and you stop the movement, this wall be deleted.