Bring up the region info box for the region you want to edit.
An easy way to do this is stand in the region and type
/rg info
If regions overlap, you will be prompted to select which region you want to view. Click to view the region.
Click the coordinates labelled Bounds to “select” the region, which allows you to view the bounds.
Figure out which direction you want to expand the region. You can do this by using F3 to figure out the direction you’re facing.
For example, to expand 10 blocks north, run the command:
//expand 10 north