You could customize the popup location by passing a pair of coordinates to
the ShowPopup() method that determines where the calendar will be
displayed. The example obtains a reference to RadDateInput's text box
and displays the popup above it by using the PopupContainer's height to
calculate the position. You need to register the PopupAbove() event
with DatePopupButton’s click attribute in order to get it execute on
clicking the DatePopup button.
Here is the forum link that describes how to show popup above the dateinput.
Popup position for RadDatePicker
Modify the coordinate values according to your requirement to show the popup to show in desired position.