You can create a custom pixel aspect ratio in existing documents, or delete or reset pixel aspect ratios previously assigned to a document.
Assign a pixel aspect ratio value to an existing document
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.
Create a custom pixel aspect ratio
Delete a pixel aspect ratio
Reset the pixel aspect ratios