With the cursor in the Diagram Viewer window, press and hold the CTRL key. You will see the cursor change to a zoom-in symbol. Press and hold the
CTRL and
SHIFT keys together and the cursor will change to a zoom-out symbol. Press and hold the
ALT key and you will see the cursor change to a pan (hand) symbol.
You can also drag out a zoom window by holding down the CTRL key and dragging out a box with the left mouse button.