What are the circumstances that led up to this change in unit?
Was the 2D drawing file created using a different unit that the 3D model? Were both files created using inches but you now need to dimension in millimeter? The reason I ask is that it might be easier to convert the units of the parts using ug_convert_part than continually changing the units of the dimensions.
That said, for existing dimensions you can right click any dimension, choose Settings and under "Text > Units" you can change the Unit. Multiple dimensions can be selected to complete this in a single operation. Going forward, you can select "Preferences > Drafting" and then "Dimension > Text > Units" and change the Unit to Millimeters. New dimensions will be created in mm.