The canvas is the entire visible area of an image. Canvas and image are independent from each other. If the canvas is smaller than the image, you can only see part of the image. If the canvas is larger, you see the original image with extra space around it.
Select Image › Canvas Size.
In the dialog that opens, enter the new size. To make sure the dimensions of the image stay the same, click the chain icon.
After adjusting the size, determine how the existing image should be positioned in comparison to the new size. Use the Offset values or drag the box inside the frame at the bottom.
When you are finished, click Resize.