How And When Did Elizabeth Learn The Truth About Wickham?

Elizabeth discovers the truth about Wickham from Darcy’s letter. The next day after Darcy’s first and unsuccessfull proposal to Elizabeth, he wrote ger a letter with the full story about Wickham: he’s a gambler, he was lying about his debt, and he tried to elope with Darcy's underage sister.
In Pride and Prejudice by Jane Austen, Wickham is quick to present himself as the perfect gentleman when he first meets Elizabeth; Elizabeth remarks that his calmness could make even the most boring topic interesting. The effort he puts into the exchange demonstrates how important it is to him that others regard him as a gentleman. This sets the stage for Wickham's lies to Elizabeth about Mr. Darcy, conveniently omitting his manipulation of Mr. Darcy and the greed that fueled his failed marriage mission. George Wickham lies to Elizabeth and omits many of the details about the truth because he wants to impress her and disparage Mr. Darcy. Wickham tells Elizabeth that Mr. Darcy’s father’s wishes were ignored when they were not. He fails to tell her about his dalliance with Georgina Darcy, his leaving his studies in both theology and law, and his mismanagement of money.
In Chapter 35, marking the turning point of Elizabeth’s development, she is exposed to the truth of Wickham’s nature and realizes that Darcy was prudent to warn Bingley of their family. From Darcy’s letter, Elizabeth learns that Wickham’s heedless action is an outcome of his independence; he gambles Julianna’s and Bennet’s titles in seek of wealth.