How can I clean up the edges of the square photo on top of this t-shirt?
To clean up the corners of a multiplied layer, use the 'Eraser' tool with a big, soft brush and gradually clean up the edges until they start to fade away. If the comp is not using layers it's more tricky, but still possible with a combination of 'Healing Brush' and/or 'Clone Stamp' tools.
If you have separate layers, I would recommend using a layer mask on that layers and using brushes (basic or textured) to mask part of it (paint with black on the mask).. Like so you can "unmask" (paint with white on the mask) and make modification to your mask easily instead of 'erasing" (destructive) parts of the real image. Make sure you have a low "hardness" on the brush and use s big brush to avoid visible edges.
Ensure the image is on a separate layer above the t-shirt image and then go to the layers palette. Top left where it says 'normal' change this to 'multiply' and toy a little with the opacity setting too. It ought to blend then with the shading in the fabric.