Yes! In the "account info" section of your user settings page, you'll see an option to set a default currency. To get there on the website, log in to splitwise.com, click on your name on the top right of the screen, and then click "My account". This currency will be selected by default when adding new bills.
Changing the default will not change the currency of your existing bills. You can change the currency of any individual bill by opening the "Edit bill" screen, then clicking or tapping on the currency symbol, which should bring up a list of all currency options. Alternately, if you would like to switch the currency on all your existing bills at once, you can do so under "Advanced features" on the account settings page of our website.