Try Like below:
Update ContractData
set ContractData.coreprocesstoken18 = udb.coreprocesstoken18
FROM ContractData AS cd
INNER JOIN
(select accountnumber, sourcesystem, effectivedate, siccode, coreprocesstoken18
from RR_GMC.RCA.dbo.UDBSourceData
where sourcesystem = 'CAN'
and effectivedate = '6/30/2012') as udb ON udb.accountnumber = cd.accountnumber