I don't see a simple way via the transform tool or otherwise to just simply straighten an image - rotate it about 1 degree or so and have it preview the adjustment.
Why is there no Help section within the interface of the app?
Please tap Transform and expand the dialog by tapping the triangle at the left.
Enter your value (1 degree) in the angle field on the right and hit OK.
Now you may want to go to the "&" menu, choose crop and trim the image.
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