The original poster was asking how to efile an amended MO return. @George4Tacks  if you go to MO, web site, it seems as they only allow mail in for amended returns. This is from MO official site, All amended returns must be completed on a paper form, Form MO-1040. There is an amended return box in the top left corner that should be checked. This is link: "https://dor.mo.gov/faq/personal/efile.php