Update only values in a field in a table where criteria is from a linked table

Question

This is the code I am trying to run, which hopefully makes sense

UPDATE OPMAT a 
INNER JOIN ARINVT b
ON a.ARINVT_ID = b.ID
SET a.EXCLUDE_REJECTS_BACKFLUSH = 'Y'
WHERE b.CLASS = 'PK'

In English ...Update the field EXCLUDE_REJECTS_BACKFLUSH from table OPMAT where CLASS from table ARINVT = 'PK' joined on OPMAT.ARINVT_ID = ARINVT.ID

I can do the is MS SQL but cant figure it out in Oracle. Currently getting the error missing SET keyword.