Something like shown below-- see if the record exists in the table.
select * from billing.acct
where accountnumber = '1234' and finalbillflag = 'F' and credflag = 'N';
-- update columns in the record.
update billing.acct set
closeoutdate = '20121231',
closedby = 'tahseenm'
where accountnumber = '1234' and finalbillflag = 'F' and credflag = 'N';