In SOLIDWORKS 2015, there are two methods to change (flip) an angle dimension.

Vertically Opposite Angle

You can now flip any placed angle dimension to its vertically opposite angle. This is useful when you wish to place the entire angle dimension outside of the model edges.

Right-click on the angle to bring up the right-click menu. Select Display Options, then Vertically Opposite Angle.

Explementary Angle

You can now flip any placed angle dimension to its explementary angle. Here is a way overly complex video about explementary angles. Here’s a simpler explanation straight from the dictionary.

Right-click on the angle to bring up the right-click menu. Select Display Options, then Explementary Angle.

Choose Explementary or Vertically Opposite in dimension preview

When creating an angle dimension with Smart Dimension tool, you can now choose between the explementary angle or the vertically opposite angle during the preview by holding down the ALT key when the mouse is in the vertically opposite region. In SOLIDWORKS 2014 and prior, you were only offered the explementary angle.