From my experience you can only edit photos before you upload them to your family tree, but if you just edit your photo one whatever app you open photos on, you should be able to save a copy so that you don't adjust the original and add it to your tree as normal.
If you need help changing a photo:
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select the person whose photo you would like to change.
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the photo currently associated with the person selected should be displayed at the top of the sidebar on the left.
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left click on the photo itself (it doesn't matter where, as long as you click on the photo).
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at this point, where the photo used to be, this should be shone:
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proceed from here as if you are adding a photo for the first time.
