My son accidentally indicated that no one could claim him as a dependent on his return. How does he amend this for 2018? I looked at the 1040X form, and it is not clear.

I cannot find anywhere in the 1040X that specifically says how to change this. It looks like it says you can change it, but not for 2018? What am I missing? Can someone give me specific instructions so I can tell him what to do?