The main guideline is to remain true to the spirit of the original post. Adding new information you think they know is usually wrong. Improving grammar, spelling, punctuation, or REMOVING CAPS is usually right. Changing posts from
markup to using our native backtick or four-spaces code markup is usually right. Changing 'i' to 'I' is usually right, but don't bother if that's the only improvement you can find. :)
As @Won't points out, your edits will go into a Suggested Edits queue to await moderation by higher-reputation users. Please don't think of this is an annoyance for anyone -- in the time it would take me, personally, to improve one post, I can vote Accept or Reject on a dozen suggested edits. (I've never seen a suggested edit take more than an hour to be Accepted or Rejected, but five minutes is more usual.)
Please keep the edits coming.