You can use a global variable to store the date that is bound to the Default property in the date picker, and in the prev/next buttons you'd decrement/increment the variable value.
In the app's OnStart property (you can find it on the first screen of your app), you'd need to initialize that value:
<first screen>.OnStart: Set(defaultDate, Today())
Now you can add the two buttons (prev/next) and set their properties to update the value of that property:
Button1.Text: "Previous day"
Button1.OnSelect: Set(defaultDate, DateAdd(defaultDate, -1, Days))
Button2.Text: "Next day"
Button2.Text: Set(defaultDate, DateAdd(defaultDate, 1, Days))
In the date picker, you then set its Default property to the "defaultDate" variable, so that when that property is changed (by clicking the previous/next buttons), then the date displayed in the picker will be changed as well.
DatePicker1.DefaultDate: defaultDate
The last thing you need to do is to update the "defaultDate" variable when the user selects a new date in the picker's calendar (so that the variable reflects what the date picker is showing, and incrementing / decrementing the date will produce the correct result):
DatePicker1.OnChange: Set(defaultDate, DatePicker1.SelectedDate)
Hope this helps!
