Photoshop Elements :: Can't Uncheck Constrain Proportions

New user of Elements 10. I am trying to change an image size but NOT keep the current height/width ratio.
 
When I go into the Image Size option box, the Constrain proportions box is ticked, so when I change the width, the height automatically alters to a new figure (say 100 x 50 becomes 200 x 100)
 
What I need to do is to have different sizes (say 100 x 50 to become 200 x 160). I realise that the image will look wrong, but this is not a problem, in fact it is what I am trying to achieve.
 
What do I have to do to be able to uncheck this option within the Image Resize dialogue box.
 
In a previous version of Photoshop, that I had access to, this was achieved by simply un-ticking the option.