Generally you can edit anything to make it clearer.
See also these Help topics ...
... perhaps especially, What is the etiquette for modifying posts?
The main restriction is, I think, is that you should avoid making edits which contradict what the author wants to say (this is sometimes called "a hostile edit").
For example if I post that the moon is 38000 km away then that's probably a typo and I'd like you to edit to correct that, i.e. I'd agree that the correction would improve the quality of my post.
Or (I can't think of a good example but) if I say that the moon is made of cheese then perhaps that's my actual opinion and/or what I really intend to post, and I wouldn't welcome your editing it. In that case maybe a comment might be better than contradicting the OP -- like, "I don't think it's made of cheese", or perhaps better a question, "why did you say that?", or, "isn't it iron-rich rock, like the Earth is?"
Generally though if it's a minor edit then that's welcome.
If it's an answer then another option is to post another (less misleading) answer of your own.
If it's a question then another option is to say "I don't understand this question".
Sometimes too it's because they misunderstood something that the author posted a question, and in that case the appropriate response might be to answer the question instead of editing it (but beware Answers vs Advice i.e. don't post an answer like, "that's the wrong question" without answering it, and also maybe avoid posting a comment which answers the question).