Once you have been given 5 Best Answers by other people, you gain the superpower to manipulate Best Answers.
However, don't.
The rules are quite simple:
1. If the thread originator has chosen an answer you leave it alone, period.
2. If you come across an obviously abandoned thread (2 or more months old) with no chosen answer you may choose one.
3. If you come across a thread with an answer that is obviously wrong, or a better one exists, you may change it, unless the answer was chosen by the thread originator, in which case you abide by rule #1.
4. You do not choose/remove an answer in an active thread unless you are the thread originator.
5. Do not use multiple accounts to award best answers to one another. You will be permanently banned for this.
Basically, leave it alone