Once you log in your NorthStar Bets go to "Settings". “Settings” can be located above your Betslip/My Bets on the right hand side your screen. You can choose between Decimal, Fractional and American odds. Once changed you will no longer have to change it after you have logged out.
For the Northstar Bets App, the odds change options is at the bottom of your Sportsbook page, so all you need to do is scroll down.