There are two ways to change the canvas size.
In absolute method you put the entire size of the image. If you start with a 4in by 6 in canvas and want a 8in by 10 in canvas then you would put 8in by 10in which would result in an increase in the canvas size. The background color will appear.
In the relative method you would check the box name relative. Then you would only enter how much you want the canvas to increase by. Therefore if you have a 4 by 6 image again and you want to increase the borders by .5 inches then you would say1 in for the width and 1 in for the height.
You can also just add canvas to one side by changing the anchor.
Changing the canvas size adapts to increasing as well as decreasing. Check out the video below.