-- SQL Server
UPDATE Sales_Import
SET Sales_Import.AccountNumber = RAN.AccountNumber
FROM Sales_Import SI
INNER JOIN RetrieveAccountNumber RAN
ON SI.LeadID = RAN.LeadID;
-- MySQL & MariaDB
UPDATE Sales_Import SI, RetrieveAccountNumber RAN
SET SI.AccountNumber = RAN.AccountNumber
WHERE SI.LeadID = RAN.LeadID;

UPDATE YourTable 
SET Col1 = OtherTable.Col1, 
    Col2 = OtherTable.Col2 
FROM (
    SELECT ID, Col1, Col2 
    FROM other_table) AS OtherTable
WHERE 
    OtherTable.ID = YourTable.ID

UPDATE accounts SET (contact_first_name, contact_last_name) =
    (SELECT first_name, last_name FROM salesmen
     WHERE salesmen.id = accounts.sales_id);