If you need to let IRS know that you now have an SSN instead of ITIN, send this letter to IRS.
Make a copy, modify it, print it in a legal-size paper (or any paper), put it in an envelope, write your address and IRS address in the envelope, put a stamp, and send it through post-office / mailbox.
Internal Revenue Service
P.O. Box 149342
Austin, TX 78714-9342