I'd just go to Image>Image Size and adjust it there. You can change the size and DPI there. However, since the image is 1200x1000, it's not going to be 1400x1400 since the original image isn't square. So if you want to make it square without distorting it, I'd change the smaller number to 1400, then I'd go to Image>Canvas Size and change the larger number to 1400, which keep in mind, would crop the image. If you don't want to Crop, but want to distort it, when you change the Size in Image Size, Click the litle chain link image to disconnect the 2 numbers and then change them both to 1400.
But keep in mind, since the imge is 1200x1000. There is going to be no way to get it to 1400x1400 without cropping or distorting it in some way
... View more