Does the United Nations enforce this declaration for free and fair elections?

The UN can monitor elections if asked by the government holding them or in extreme cases when a territory is administered by the UN as was Kosovo after 1999. UN or internationally monitored elections increase the cleims to legitimacy of the resulting government and reduce the likelyhood of post-election claims for fraud and possible violence. The declaration quoted by you has moral value and will be sued by pro-democracy campaigners but is not automatically backed by force if disregarded.