Desktop: by selecting either fractional or decimal from the black banner at the bottom of the page. Mobile: by clicking on your balance and selecting 'Settings' and 'Odds Display'.
Fractional odds represent the profit - 5/1 means you will win five pounds for every one pound staked - whereas decimal odds represent the return - 2.2 means you will receive 2.20 for every 1 invested, including your stake. So, think of decimal odds in monetary terms.