The Zoom tool is designed to change the image scale. Press Z to quickly access this tool.
To zoom in left click on the image. To zoom out left click with Alt on the image.
It is also possible to increase the scale of the selected area. For this purpose press and select a rectangular area. When the rectangular area is selected, the image scale will be changed so that the selected part of the image will fill the whole window.
Double-clicking on the tool's icon on the Toolbar makes the image scale to 100% (Actual Size).
Even if another tool is active, you can use these hotkeys:
to zoom in: + and Ctrl++ on Windows, ⌘++ on Mac,
to zoom out: - and Ctrl+- on Windows, ⌘+- on Mac.