It is not possible to change the casino account currency after the registration has been completed. Some casinos select the best currency based on your geolocation, while others let you select your preferred currencies from a list of supported currencies.
Not every casino accept every possible currency, so if you wish to play with a currency other than US dollar or euro, which are widely accepted, you have to look around a bit more. You can find casinos with specific currencies on this page.
If you wish to change the account currency, you will need to contact the customer support of the casino, to discuss the necessary steps. Under no circumstances, we recommend just registering a new account, as most online casinos don't allow more than one active account at a time, and they can void winnings achieved with your new account.
The customer support can close your current account first, and you can then register a new player account with the new currency at the casino.