Operating the mouse wheel while hovering over an empty part of the canvas, e.g. no canvas element is highlighted at the current cursor position, will zoom the canvas towards/away from the current mouse cursor position.
Zooming in/zooming out is also possible by clicking on and buttons at the bottom right of the canvas view.
Clicking on the Zoom to fit button at the bottom right of the Canvas will switch between a magnification, which displays the entire molecule in your current viewport and the previous zoom level.