undeceive

Etymology

From un- + deceive.

verb

1. (transitive) to free from misconception, deception or error
   […] Marcasites, I say, being thus fitted to delude the unskilful, I have had much ado to undeceive some, that brought or sent me them from America, of the pleasing Confidence they had entertained, that these promising Fossiles were Lumps of rich Ore of Gold, or Silver. 1690, Robert Boyle, “A Previous Hydrostatical Way of Estimating Ores”, in Medicina Hydrostatica, London: Samuel Smith, Section V, p. 168
   If you think that this loss has broken my heart undeceive yourself, for such as I live years in an hour and show no sign. 3 January 1863, Louisa May Alcott, chapter 1, in Pauline’s Passion and Punishment: Frank Leslie’s Illustrated Newspaper, volume XV, number 379, page 229