You can move a dimension line using the Move Dimension Line tool. When you move a dimension line, the extension lines stretch to accommodate the move.
To move a dimension line using the Move Dimension Line tool:
Select the dimension.
Right-click and select Move Dimension Line, or select Edit > Modify Elements > Move Dimension Line.
Click any point to serve as the base point for the move.
Move your mouse in the direction you want to move the dimension line.
Click to finish the move.