With
a document open, choose Image > Pixel Aspect Ratio and
then choose a pixel aspect ratio that’s compatible with the video
format that you’ll be using your Photoshop file with.
With the document open, choose Image >
Pixel Aspect Ratio > Reset Pixel Aspect Ratios.
In the dialog box, choose one of the following:
Append
Replaces the current pixel aspect ratios with the default
values plus any custom pixel aspect ratios. This option is useful
if you deleted a default value and want to restore it to the menu
but also want to retain any custom values.
OK
Replaces the current pixel aspect ratios with the default
values. Custom pixel aspect ratios are discarded.