Welcome to the forum. As I'm sure you know, SW should default to a radius dimension for arcs and diameter for circles. If they aren't, then I'm not sure why or how to fix it. However, you can change the display type in at least two ways.
1. Click on an extension line or leader line of the dimension (or on the dimension itself if you don't have Instant 2d activated) to bring up the dimension's PropertyManager. Go to the Leaders tab and choose the desired display style. Or...
2. ...Right-click on it and choose "Display Options > Display as Diameter (or Radius)" from the drop-down.
Thank you, on both counts.