It's done with the ruler setup...
From the Corel 12 Help:
To customize ruler settings
Click View > Grid and ruler setup.
In the Document list of categories, click Rulers.
In the Units area, choose a unit of measure from the Horizontal list
If you want to use a different unit of measure for the vertical ruler,
disable the Same units for horizontal and vertical rulers check box,
and choose a unit of measure from the Vertical list box.
This may change for just this document... do the same under Tools >
Options to make it permanent (Not positive but probably)