Switch the drafting standard to ANSI.
Two screenshots showing the difference are attached. ANSI places all dimensions horizontal. ISO allows both horizontal and vertical dimensions.
Answered with a tutorial: https://grabcad.com/tutorials/how-to-change-the-dimension-making-it-not-vertical
Click on the line. A green box with a vertical line should appear. Click on this box and press the delete button on your keyboard. This will get rid of the vertical relationship and allow you to put the line at an angle.
Shaun, Your answer applies to changing the orientation of a line, but Ahmed was looking to change the orientation of a dimension.