Change content type

Content type is document and I cannot find the option to add the "Folder content type".

When I open the form the content type that is showing is the folder content type.

and when I try to add the custom column to that content it is taking too long to add that field.

What im trying to do is add a field in the properties form called Project Manager but it only showing 3 fields : content type with default value folder, title and name.

Im tried the steps above but i lt didn't work for me.