It took me a while to find such a perfect example. Judging by the first look, cleaning this is simple. Yet I see this being
cleaned not as perfect as I hoped it would be.
Step 1 --> Select with your 'Polygonal Lasso Tool' this area.
Step 2 --> Hit on Ctrl, C (copy) Step 3 --> Go to File --> New. Step 4 --> Hit on Ctrl, V (paste)
Step 5 --> Go to Layer --> New Adjustment Layer --> Brightness/Contrast.
Step 6 --> Use a brightness of 10 and then click on 'Ok'.
Step 7 --> Go to Layer --> Merge Visible Step 8 --> Select the layer and hit on Ctrl, C. Step 10 --> Select the window of the scan and hit on Ctrl, V. Step 11 --> Click on the move tool and move the layer exactly like the below given example.
Step 12 --> Erase a part of the layer (near the bubble) so everything is looking fine. Step 13 --> Flatten the Image.
As you already figured out, you can also do this with the 'Healing Brush Tool'. DON'T~! Learn it also this way.
Why? Because there are situations where you can't use the 'Healing Brush Tool' and you really need to apply this technique.
When you got the hang of this technique, you can finish it off with a soft-rounded healing brush.