These kinds of questions are better answered from Meta Stackexchange. I did a search to find what you're asking about and this link will take you to the results.
I did find a few off the top of the list which I believe sort of respond to your overall question. Those questions are:
The top one is more a generic question about answering, while the second one is more pointed towards what you are asking (but might not quite cover it). There is a 15 wait after a question is asked before an answer can be selected by someone other than the original poster of the question. There is also a (IIRC) 48 hour waiting period for an answer posted by the original poster of the question.
What it really boils down to is this: If you wrote the question, it's your call what you feel works best for you and then select that as you desire.
As someone looking to answer a question, if you feel like you have a better, more complete, or more accurate answer, nothing is stopping you from adding the answer to the question whether or not something has already been selected as "the answer". It is then up to the community to upvote it and up to the OP as to whether they want to select your answer as better than the original.
To me it's disheartening to see the first answer getting selected as "the answer", especially when the answer is really not a very good answer, doesn't really answer the question, and doesn't have any votes. IMHO, people are just too quick on the draw to click the checkmark. There is only one way we can hope to convey to either the OP or the answerer the information provided is bad and that's through a downvote. I personally don't like downvotes, but I'll pull them out if I need to. I highly recommend anyone else do the same if they feel information is bad, wrong, or could cause someone bodily harm.
That's about all I have for you. Unfortunately there isn't much we can do about it, but if you feel so inclined, please ask your question over on Meta where it can garner a lot more attention than it ever would here on our little Stack of the world.