HOW DO I DO A MASS ACCOUNT OWNER UPDATE?

HOW DO I DO A MASS ACCOUNT OWNER UPDATE?

BECAUSE I UPLOADED ALL OF OUR ACCOUNTS (20K RECORDS) IT ASSIGNED ME THE AS THE ACCOUNT OWNER. I WANT TO HAVE A RULE THAT MATCHES THE CREATED FIELD IN THE ACCOUNT MODULE (SALESPERSON) TO TRIGGER THE RESULT TO A CORRESPONDING USER, HOWEVER, THE RULE DIDNT WORK.

THOUGHTS?