Who delivers the news of Duke Frederick's conversion?
You should refer to the eNotes study guide for this sort of information. The news of Duke Frederick's conversion is brought by Jacques de Boys, the middle son of Sir Rowland de Boys, father of Orlando. Jacques de Boys is a very minor character in the play.
The following is an excerpt from the eNotes study guide for Shakespeare's As You Like It, which provides an excellent and thorough summary of the entire play.
The next day, Rosalind reveals her true identity; and she and Orlando, Oliver and Celia, and Silvius and Phebe are married before the banished Duke. Jaques de Boys, the middle son of Sir Rowland, brings the news that Duke Frederick has met an old religious hermit and has decided to forsake the world and restore his brother's dukedom. The newly united couples dance, and Rosalind speaks the epilogue.