Make sure you keep a backup of your model before you attempt this.
Assuming that your model has only members (no plates or solids):
Stay in the modeling mode
Select all the members
Go to the Geometry menu on top of the screen
Choose Rotate
In the dialog box that comes up, specify 2 points that define the axis of rotation. In your case, since it happens to be parallel to the global Z axis, and assuming that it passes through the origin, specify the points as (0,0,0) and (0,0,10).
Specify the angle of rotation as 90 degrees. Set the Generate mode to "Move".
Click on OK.
Another way is to add the command
PERFORM ROTATION Z 90
in the input file at the end of all the MEMBER INCIDENCES.