Currency is automatically selected for you, based on your location.
For example: if you are located in Canada, you will be charged in Canadian dollars.
If you are in the USA, you will be charged in US dollars.
The currency cannot be updated manually, it is an automated process. By charging you in the currency of your billing country, you avoid having to pay foreign transaction fees that your credit card/bank charges, reducing what you would end up paying if you were charged in USD.