MySQL update table based on another tables value

UPDATE TABLE2
       JOIN TABLE1
       ON TABLE1.SUBST_ID = TABLE2.SERIAL_ID
SET    TABLE2.BRANCH_ID = TABLE1.CREATED_ID 
WHERE TABLE2.BRANCH_ID IS NULL or TABLE2.BRANCH_ID='';

In addition to Tom's answer if you need to repeat the operation frequently and want to save time you can do:

UPDATE TABLE1
       JOIN TABLE2
       ON TABLE1.SUBST_ID = TABLE2.SERIAL_ID
SET    TABLE2.BRANCH_ID = TABLE1.CREATED_ID
WHERE TABLE2.BRANCH_ID IS NULL

UPDATE TABLE2
       JOIN TABLE1
       ON TABLE2.SERIAL_ID = TABLE1.SUBST_ID
SET    TABLE2.BRANCH_ID = TABLE1.CREATED_ID;