If your Navigator palette isn’t already visible, choose it from the Window menu in the Photoshop toolbar that runs across the top of your program’s window. When your Navigator palette is open, look just below the mini image of your photo.
You’ll see a percentage next to a slider, which is your Zoom tool. Move it to the right to zoom in. If you need to adjust where the tool zooms, hover your mouse inside the red frame that appears in your mini image. Then you can drag the frame to the position you need.