You have to use an exchange like Coinbank or Bitfinex, they'll take your bitcoins and offer you whatever other currency. Thing is that these exchanges often have restrictions on when, how often, and how much you can exchange, often they have queues that you have to wait in to actually exchange your currency. If you're a new user, you can usually expect to wait a while to drop your bitcoin for some other currency, and you can usually expect to have a limit on the amount you're allowed to convert. The exchanges do this to help prevent a crash via a flash selloff, and to some extent it's successful, however eventually consumer faith in the future of the value on Bitcoin is going to drop off (I mean really, $17k?) and when that happens those same protections are going to make the crash all that much worse for people involved.