I have earned enough nxt. How can I withdraw and convert it into btc?
According to their FAQ, payouts are processed every Sunday:
When will i get paid?
Right now you will get payed every Sunday.
Click left on "Refer" to see the actual countdown. Due to the high fees for every Nxt Trade, payout minimum is 1 Nxt. This will be adjusted in the future, when divisibility is added by Nxt.
Link:
http://freenxt.org/?menu=faqClicking on "Refer" shows me this:
Payment will be made when you have more than 1 Nxt.
You needed to enter your NXT address in order to sign up in the first place so I would assume this is where payments would go. Once the coins are in your wallet, you can send them over to an altcoin exchange such as Cryptsy and exchange them for BTC.
If you don't want to bother with making an account then you can try
ShapeShifter.io to exchange your coins as well.