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Hmm yes, step #1 is converting SBD into bitcoin/other cryptocurrencies that you can then sell for serbian dinars/euros/etc (fiat currency).

You can convert your steem dollars (SBD) for any cryptocurrency you like, including bitcoin, using either an exchange or a website like blocktrades. The advantage of using an exchange is that the fees are lower, however it's more complicated to use, while on websites like blocktrades the fees are higher, but it's extremely easy to use.

Then you have to find an exchange that allows you to sell your cryptocurrencies for serbian dinars or some other fiat currency... sadly for that I can't help you, since I only know exchanges that trade in US dollars or Euros.

In total you will have to pay 3 "transaction fees" :

  1. first conversion fees (SBD->bitcoin)
  2. Transaction fee for sending bitcoin from 1 address to another address (from the first exchange to the second one)
  3. second conversion fees (bitcoin-> serbian dinars/USD/Euro)

be careful about them, for step 1 blocktrades in particular has crazy high fees imo.

Bitcoin transaction fees are pretty high too, use another crypto if you can (btc fee for 1 transaction: 10USD, while Litecoin transaction fee: 0.25USD (https://bitinfocharts.com/comparison/litecoin-transactionfees.html).

Ok but also I don't know how long I can keep SBD in my walet. Is need to make transaction each 7 or 10 days?

No, no need to make any transactions, you will not lose your SBD. You get your rewards with a 7 day-delay, then they are yours forever

this link should help you : https://steemit.com/faq.html#How_can_I_withdraw_my_STEEM_or_SBD_coins

I'd recommend you read the whole steem FAQ if you have the time