Here are the instructions in the Help:
To move an ordinate origin:
Double-click a dimension in the set to open the Ordinate Dimension dialog box in the edit mode.
In the References group, click Select Origin .
Select the new origin point.
The origin moves and all of the dimension values are recalculated from the new origin.
double click one dimension.
the ordinate dimension dialog box pops out.
you should see a broken line in the graphics window, see the image for reference. here I have vertical and horizontal margin.
i marked it with a blue pen.
rightclick one of the broken lines, and select move margin. (in my case i choose the vertical one)
change the value of your offset to the value that you need.
that would move your first margin and do the same on the other one.
hope this helps