Open Settings and choose General. The Clock card has two choices: 24-hour, which shows 13:30, and 12-hour, which shows 1:30pm. It saves as soon as you pick one, and it is for your whole business, so everyone in your team sees the same.
Your diary follows it - the hours down the side, each booking, the booking panel and the clock at the top - and so do your dashboard, Analytics, messages, each customer's page, the activity feed, your invoices and quotes, and the new-booking emails and alerts DocMason sends you.
What your customers see stays on the 24-hour clock: your booking page, and the emails and texts they get.