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;