Right click the "model" at the top most level of the scene manager tree (usually the right most pane).
"Move" the model
A triad/control wheel will pop up on the screen, just select the axis and type of adjustment (rotation, translate, scale) to perform, and you will get it.
It may be helpful to choose the "right" or "front" camera to get a good sense of when it is straight.
You can try "snap to ground" when done, but I usually have to slide the part up/down to the right elevation.