Hold the left-hand mouse button down and move the cursor left and right to rotate the current view in the XY plane. To rotate the view in the vertical plane hold the left hand mouse button down and move the mouse up and down.
Hold the right hand mouse button down and move left, right, up or down. Alternatively, you can use the keyboard cursor keys (up, down, left and right) to move the image around on the screen.
Use the middle mouse wheel to zoom in and out. If you need to zoom-in, rotate the mouse wheel towards you. To zoom-out rotate the mouse wheel away from you.
Use the keyboard command Ctrl-R or the main menu option View/Reset view to visible meshes to reset the current view to that first presented to you. The default view is aimed directly at the center of all visible meshes.
To customize the sensitivity of the mouse use Tools/Options … from the main menu.