If your question is very close or essentially the same, and the answers aren't good, either edit it to increase clarity or offer a bounty (which may not workmay not work but is worth a try).
If your question is a twist on or a followup to the earlier question, ask a new one and link to the old onenew one and link to the old one as part of your question.
If the existing question is general but you have a specific case not covered by existing answers, offer a bounty for improved answers which cover that case too. Or, if there's a good reason the special case should be considered separately from the general — either your case is too weird, or else the general is too general — then a followup question is appropriate.
In the other direction, if the existing question is very specific and you want to generalize, I suggest looking at the existing answers. If they are steering very strongly towards the general (because the question reasonably was made more specific than necessary, like asking for brand-specific solutions to non-brand-specific problems), then editing the question to also be more general might be okay (used with caution). If the question is specific and the answers are correspondingly targetted, a new question referencing the old one is definitely in order.
