Method 1: Using the Blue Handles
Double click on the image and use the blue handles to manually adjust the aspect ratio.
Once you chose the desired ratio, press enter on your keyboard or click outside of the image to apply it.
Method 2: Using the Ratio Dropdown
Under the Document tab in the left-hand panel, click on the Ratio dropdown and choose your preferred aspect ratio.
If you want to revert your changes, click the Revert button (this will also revert crop).