How do I change my gender on honesty box (facebook app)?

I want to change it from female to the 'gender unknown' like to the grey color.How do I change my gender on honesty box (facebook app)?
on the left under ur current avatar click on the change button and you will have to pick one of those because i dont think you can get the grey on back after you have changed it.How do I change my gender on honesty box (facebook app)?
i dont know if you can but if there was a way you would have to click the charecter to change the settings