Bitcoin is already a currency, it does not need to become one. You can buy and sell things with bitcoin and that is what a currency is for. Although now I think people use bitcoin as more of an investment than as a currency but that does not mean that bitcoin has stopped being a currency.
Yes, you are right because nowadays you can buy some stuff using bitcoin or any crypto currency , but i think the OP want to say is how bitcoin become a fiat currency. Anyways, bitcoin is a digital currency that we can use to store, transact, exchange and pay, etc.