If there is an old question that has received no or inadequate answers, it is likely not going to get an answer without intervention, as it is not appearing high enough on the lists of new or unanswered questions to get the attention of people who may be capable of providing an answer. If someone wants to see a particular question answered, what are the different methods they can use to get it the attention it needs? Is there a way to bump questions up these lists? Are there different methods available for those who are the original poster vs those who aren't? Are there any rules of etiquette that should be adhered to as well?
The right answer would be, as several comments say:
adding a bounty to the question, and
significant improvements to the question (if any exist).
It is questionable what to do with a question that has some answers, and they are all inadequate. It might be best to flag/downvote the answers appropriately, adding a clarifying comment, since the answers are the real culprit, and leave the question as it is.
As opposed to the above, certain actions could go against you in various ways:
making insignificant edits simply for the purpose of bumping, and/or
posting a new, duplicate question (whether it points to the old one or not).
Such actions could potentially be considered as harrassing the community and/or abusing the site, and could hypothetically lead to the community reacting via downvoting, question closing, putting the question on hold, question deletion, etc. Your reputation could suffer. Instead of 3. or 4., it is probably better for you to leave the question as it is or even remove it altogether (if possible) to let the search engines search elsewhere for solutions in the long run.
This might sound strange, but just upvote the existing answers!
If the authors are still around this works remarkably well to trigger an update of outdated answers because many of the users will have a look at their post if they receive an upvote to an very old answer and sometimes within a couple of minutes they will update an outdated post.
Even if this does not work, it certainly is worth a try - you can't lose anything.