If you're Unverified, you can update it by submitting a new BOP Request form.
If you're Pending Unverification, wait until you're Unverification is complete, then follow instructions for Unverified.
If you're Verified, you must create a new account with the new SSN. SSN cannot be changed for a Verified account. If you then wish to transfer data from the account with your previous SSN to the new account, you can request this change by