De-link old account or link new account

Signed up for service for the first time in at least half a decade if not more and it gave me the last 4 digits of a new acct number and I got my modem today.  But the app is showing my old service address 3 states away from where I live now and old account number and no service.  I need my

old account unlinked from my info and new account linked so I can set it up and manage it.