Resizing a file in Photoshop CC - no constrain proportions box?

I'm a complete novice to Photoshop CC and I feel like I'm missing something very obvious, but I can't find the answer. I want to re-size my 1024 x 748 image to 1024 x 768. When I look online, it simply refers me to the "Constrain Proportions" option box to accomplish this - for example, like in this pic:

However, when I go to the Image Size option, all I get is this:

I have no option to constrain proportions or not, and if I try to resize my height to 768, for instance, it tries to scale the width along with it, which I do not want and stretches the image. Am I missing something very obvious? Thanks.