The fist what should be said is that SketchUp model is not driven by dimensions which are setup by "Dimension" tool - those dimensions are just text and arrow objects, so one can click the text and change it to any other text not even related to any dimension. Furthermore, in most cases you really do not want to modify them this way - such a change will kill the possibility of the dimension value to track the true size.
The unpleasant thing about changing dimensions is that it should be done different way for different shapes:
- For a line segment - right click on it and choose "Entity Info"
Type a new value in the "Length" field, hit "Enter" and that will make the change.
- For 2D shapes - select a 2D face with "Scale" tool
then select a green box which corresponds to the dimension you are intending to change
The "Red Scale" field ("red" indicates that the scaling is along the red coordinate axis) in the right bottom corner of the SketchUp window by default is in scale mode. Just type a new exact dimension with a unit (1m in this example)
and press "Enter" - the dimension will change and the value will follow.
- For 3D shapes (mostly the same as for 2D case) - with "Select" tool select a whole shape
switch to "Scale" tool
and select a green box corresponding to the dimension you're going to change.
Type a new exact dimension with a unit (1m in this example)
and press "Enter" - the dimension will get changed.