* Medium: Refers to a single means of communication, expression, or artistic creation (e.g., "paint is her preferred medium," "television is a powerful medium").
* Media: Refers to multiple means of communication, expression, or artistic creation (e.g., "social media," "the news media," "various media outlets").
Therefore, you can't *always* write "media" as "medium." It depends on whether you're referring to one or many. If you mean more than one, you *must* use "media." If you mean only one, "medium" is correct.