Some errors just keep sticking around. Supercede is one of them.

Supersede is the correct spelling. The verb – meaning to replace or substitute – comes from a French word, which came from a Latin word, and both of those languages also spelled it with an s.

As with modern English, some French and Latin writers of the past misspelled it with a c! In English, several other words end in –cede rather than –sede, so at least modern writers have an excuse for making the mistake.

Because the error has been so pervasive for so long, a few dictionaries now list supercede as acceptable. To be strictly in the right, through, use an s.