You can't set the date format directly. It's determined by the Culture you select in your profile: each culture comes with a fixed set of regional formats (date, time, and decimal display), so you change the date format by choosing the culture that uses the format you want.
To change it:
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Open your profile and go to the Regional settings tab.
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Open the Culture dropdown and select the culture whose formats you want. For example, "English (United Kingdom)" uses the date format dd/MM/yyyy.
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Click Update profile.
The Date format, Time format, and Decimal display fields below the dropdown are read-only. They preview the formats tied to the selected culture and update automatically when you change it. This setting is per-user, so it affects only your own view.