The official spelling is supersede. This verb means to replace or substitute something for something else. It is a transitive verb that is used with an object.
Supersede comes from French, and then Latin before that. In both languages it is spelled with an s. However, the misspelling supercede has been recorded for multiple centuries. Because of the pervasive use of this error, supercede is listed in most dictionaries. These entries simply refer the user to the correct spelling. It is interesting to note that the error has never been adopted as an accepted alternative, which is the case with some other widespread errors.
The noun form is supersession.
A person may be a superseder. This word is officially listed in reputable dictionaries, but is not used very often.
Supersedable is a slowly growing adjective that describes something or someone as capable of being replaced or substituted. It is listed in several dictionaries; however, is not used much in formal writing and should probably be avoided for now.
Equally we were looking at the natural order of generations, and the inevitable role of the child to supersede their parent. [The Guardian]
In respect of the Urban Co-operative Banks (UCBs), issues relating to appointment of professional directors on the boards of UCBs, delay in appointment of liquidators by Registrar of Cooperative societies (RCS) in respect of UCBs whose licences were cancelled by the Reserve Bank, delay in supersession of board/appointment of Administrators when requisitioned by the Reserve Bank, connected lending, etc., were highlighted. [The Hindu Business Line]