Can't say much without actually seeing your sample data. I'm not even sure of the table names you have used in the query...but this should be a start..
UPDATE l
SET label = 'By The Numbers'
FROM label l
INNER JOIN (SELECT rep_id,
trade_type,
trade_year,
tran_sum_month,
firm_id,
terr1,
Sum(trans_value) AS trans_value
FROM tran_sum_month
GROUP BY rep_id,
trade_type,
trade_year,
tran_sum_month,
firm_id,
terr1) t
ON l.contact_id = t.rep_id
WHERE t.trade_type <> 'd'
AND t.trade_year > 2007
AND t.firm_id NOT IN ('FHO05028','FHO05948','F 00630','FHO06058',
'FHO05585','FMB07822','FHO04974')
AND t.terr1 <> 'UN'
AND t.trans_value > 35000