deselectItem

Method which deselects the table row passed as an argument or the row corresponding to the index passed as an argument. If you want to deselect all rows (meaningful with multi-row selection enabled), execute the clearSelectedItems() method instead.

deselectItem (gridItem) or deselectItem(index)
row	HTML row object -
The row that will be selected.
index	Integer	The row corresponding to the index will be deselected.

Example:

function DeselectFirstItem() {
    var masterTable = $find("<%= RadGrid1.ClientID %>").get_masterTableView();
    masterTable.deselectItem(masterTable.get_dataItems()[0].get_element());
}