No, you cannot change a bounty once it has been allocated.
But why would you want to? What's the hurry in assigning the bounty to an answer? The whole point of raising a bounty on your question is to attract attention, and hopefully good answers. There's no reason to settle for the first one that rolls in; give it time and see if you get anything better. At the end of the bounty period, you can look at what answers you have and choose the best/most helpful one.
It's not like the person is going to delete their answer in the meantime, so what have you lost by waiting?
Technically, you could go ahead and assign the bounty to this answer, and then if another answer is posted, you could start a second bounty to reward that answer. But this has a couple of problems. First, once the bounty is assigned, you lose the major advantage of having an active bounty: your question appearing in the "featured" tab and attracting the attention of answerers. Second, the starting amount for a bounty goes up each time, so you'll end up having to slice off more of your reputation for this.