How does Duncan reward Macbeth for his bravery in defeating the rebels?

King Duncan rewards Macbeth's bravery with a promotion.

In Act I scene ii, a "bloody sergeant" tells King Duncan that Macbeth bravely slew Macdonwald, the leader of the rebels, by slicing him from navel to chin and beheading him: "Till he unseam'd him from the nave to the chaps, / And fix'd his head upon our battlements." Unseamed means cut open; from the nave to the chaps means from the navel to the chin; fixed his head means stuck his head; upon our battlements means on our fort's wall.

Later in the same scene, the Thane of Ross shows up and informs the king that the Thane of Cawdor is a traitor and was helping the rebels: "Assisted by that most disloyal traitor / The Thane of Cawdor...." The scene concludes with Duncan telling Ross to execute the traitor and give his title, Thane of Cawdor, to Macbeth.

This is how Macbeth realizes that the three witches' prophesy is true and later decides to kill the king. In Act I scene iii, The three witches meet Macbeth and Banquo on the battlefield and tell Macbeth that he will be Thane of Cawdor and then King of Scotland:

FIRST WITCH: All hail, Macbeth! hail to thee, Thane of Glamis!

SECOND WITCH: All hail, Macbeth! hail to thee, Thane of
Cawdor!

THIRD WITCH: All hail, Macbeth, that shalt be King hereafter!

Macbeth is currently the Thane of Glamis, but he does not believe he will become Thane of Cawdor and then king: "...to be King / Stands not within the prospect of belief, / No more than to be Cawdor" until he is informed that he has been promoted to Thane of Cawdor. In a way and without knowing it, by promoting Macbeth King Duncan has started the chain of events that will lead to his own death.