Double-click one of the dimensions.
The Modify dialog box appears.
The current dimension is highlighted.
Set the value to 120,
then click 
.
The sketch changes size to reflect the new dimension. The dimension
value is now 120mm.
Click Zoom to
Fit 
on the
View toolbar to display the entire rectangle at full size and to center
it in the graphics area.
Double-click the other dimension and change its value to 120.
Click Zoom to
Fit again
to center the sketch.
