Reparent DEF bones operators are accessible on the side bar.

To show the side bar, press N while the mouse cursor is in the 3D viewport.

The check box 'Parent to First' allows DEF bones to be parented to the First DEF bone in the Rigify segmented DEF bones chain. In most cases, it should be unchecked to allow DEF bones to be parented to the Highest Index DEF bone in the Rigify segmented DEF bones chain.

Make sure the generated Rigify rig does not use 'Connect chain' otherwise reparenting will fail.