Assuming your ComboBox index is 0, you can use the code below to set the selected item as the first item:
((contextMenuStrip1.Items[0]) as ToolStripComboBox).SelectedIndex = 0;
Or you can also access the ComboBox by key:
((contextMenuStrip1.Items["myComboBox"]) as ToolStripComboBox).SelectedIndex = 0;