Choose Tools > Zoom (from the Tools menu at the top of your screen).
Press Z on your keyboard.
Do any of the following:
In the Tool Options pane, drag the Zoom slider to zoom in or out of the image. You can also use the Navigator box to change the area of an image on which you’re zoomed in.
Click the disclosure triangle next to the current zoom to choose a default zoom percentage, or type in a custom zoom percentage into the percentage field.
Under Mode, select Zoom In or Zoom Out and click the area of an image that you’d like to zoom in or zoom out.
Click and drag on the canvas to use scrubby zoom. When a different tool is selected, hold down Option ⌥ + Space while dragging.