UPDATE with a SELECT

[John Biddulph]

I would like to run the following UPDATE:
UPDATE `servicescopy` SET `newCustomerID`=ct.userid WHERE CustomerID = cust.CustomerID

based on the following SELECT:

SELECT cust.CustomerID, ct.userid
FROM tblcontacts as ct
INNER JOIN tblclients as cli
ON ct.userid = cli.userid
INNER JOIN customerold as cust
ON CONCAT(cust.Initials, cust.Surname) = CONCAT(ct.firstname, cli.company)


Please help