I know in most places around the world, most people advice to exchange currency at one of the official bureau du change but in Uzbekistan it's best you exchange your money on the black market because you will get a better exchange rate (40%) for your bucks. Having said this, make sure your bills are in great condition and use a guide to help you do the exchange.
the most widely used currency in Uzbekistan is the U.S. Dollar so bring dollars with you. You can exchange money in most hotels, banks as well as in exchange kiosks in major supermarkets and bazaars. I wouldn't suggest doing the exchange any other way as the Uzbekistan government is clamping down on black market exchanges.
Bring as many US dollars as you can to avoid bank withdrawals, which are a headache. Also, make sure the US dollar bills are new and ‘crisp’ with no marks. If you need to change more money, I too suggest doing it at the local bank/hotel using the official exchange rate.