restore edit button

On a Mac the Edit button should be in the toolbar, if you are viewing a photo enlarged. You can also go to Edit mode by selecting the thumbnail of a photo and hitting the return button.

If edit is not available, you may have selected a photo that cannot be edited, for example, if you are viewing a shared album or if the photo is in an unsupported file formatter the original image file is missing, or the photo has been edited in Aperture or iPhoto ad migrated to Photos. If a previously edited photo cannot be edited, try to revert the photo to the original with "Image > Revert to Original".

In Photos on your iPhone or on your Mac? Below your question it is showing iMac 27″ 5K, but you are asking in the forum for Photos iOS (iPad, iPhone, iPod touch)?