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