It depends. You can:
- Create a new answer. Reference the existing one. "Expanding on Bob's answer, you should also remember to frib the noodle after sending at least three trilobytes."
- Copy the existing answer into yours, but make it clear where the earlier content came from. Blockquote formatting (with a referencing header or footer: "copied from Bob's answer") helps here.
- Is a part of the original answer actually missing - is it incomplete, as opposed to just imperfect? You can leave a comment on it, requesting expansion. If they don't respond, then try one of the above.
Be very careful before editing another user's answers. You should stay away from large changes that might change the meaning of the answer. And since you don't yet have 2k reputation on Stack Overflow, you'll be suggesting an edit, rather than editing directly - suggested edits making substantive changes are often rejected.
Good on you for wanting to fix things up!