I had asked a question 10 months ago and right now I gave bounty on that. The reason for bounty I gave was 'More detail require' But after applying the bounty it shows 'The current answers do not contain enough detail'. It contains enough details but I wanted more. Now how can I change the reason for bounty?
In general, you can't.
In this specific case, seeing that you just put on the bounty now-ish, I've refunded your rep and removed the bounty. You should now re-open the bounty and in addition to selecting the "more details required" reason, also type in a more detailed optional text describing what you are looking for in the bounty.
(Now to the general audience reading this: This is a one-time thing. I can't guarantee that I or any other moderators will be willing to refund bounties for "changing reasons" in the future. So be more careful when you open a bounty. )