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.)

I ended up adding a footnote to the page, with a numeric mark. In this, I referenced the source which explained why it makes more sense to change the words.

To preserve a comment made by Ben Bolker as an answer, since comments should not be used as answers and can get deleted at any moment:

If you want to make an editorial change to a direct quote because you want to shorten something or fix a dodgy translation, the word(s) you change should be put between [square brackets]. This is to indicate that this is a deliberate change you made to the original text.