You can't edit "within" Photos. But I think you mean edit the Photo directly from Photos, without exporting and then importing again. You do it the same way you use any other external photo editor in Photos.
Select the photo, go to Image, Edit With, and then select Seashore. Then make your changes. Choose File, Save. Wait a second or two (check to make sure the new version is in Photos) and then you can close the document in Seashore if you are happy with your changes.
You can't edit "within" Photos. But I think you mean edit the Photo directly from Photos, without exporting and then importing again. You do it the same way you use any other external photo editor in Photos.
Select the photo, go to Image, Edit With, and then select Seashore. Then make your changes. Choose File, Save. Wait a second or two (check to make sure the new version is in Photos) and then you can close the document in Seashore if you are happy with your changes.
See https://macmost.com/editing-photos-in-external-editors-2.html