I-824 approved, but didnt receive official letter due to moving

can somebody help to tell me how i can do ? i got email from USCIS , they said, I-824 approved, the letter has been sent out, but i have not received any letter so far almost 20days passed, I also changed my mailing address online and got Reference No#, in this situation, how can i do ?
i am not sure how is the following procedure to process I-824 finalized