I changed a word from a source, how do I cite it correctly?
Write:


  [...] something extended, flexible and [mutable].


You might like to elaborate in a footnote, e.g., 


  The compendium poorly translates original word as movable, but mutable is more appropriate, as noted by Marleen Rozemond in "Descartes' Dualism," page 93.


(Depending on the style, you might like to replace in "Descartes' Dualism," page 93 with a citation.)