After completing a number of jobs in Whiterun (5 or so, I think…), Delvin will have this “Special Job” for you. We are to go talk to Olfrid Battle-Born, a wealthy merchant who needs help. Go over to Whiterun and find him. He’ll tell you about his friend Arn, who is slated for execution and is in the local jail. He doesn’t want him broke out though, instead he wants us to get into the Jarl’s Quarters and steal a letter from Solitude warning Whiterun about Arn, and then changing his name in the Steward’s prisoner registry…

Head on up to Dragonsreach and enter (be sure to save). Head up past the throne (the stairs are on the right) and enter the Jarl’s Quarters through the big double doors to the left. Once inside, enter the door on the right to find the prison registry and forge the names. EASY. Now, head up the stairs and enter the middle door, then the door on the right to find the Jarl’s study. Make sure you’re hidden and take the letter on the desk. Again, easy…

Return to Olfrid and let him know you’re done. He’ll wonder who this Arn guy is that you’re talking about! He’ll also give you a Ring of Replenishing as a thank you token. Return to Delvin now and let him know you’re done.