You can update as many fields as you want by separating each with a comma, as in the following:

UPDATE PRODUCTION
SET CUSTOMER TO NULL, SLIPID TO NULL, STATUS=0
WHERE SLIPID="17753"

There are also commands to insert new records and remove records. This is a basic introduction to SQL. There are many more features that haven’t been discussed, including joining tables by selecting from more than one table, embedding select statements, searching for partial matches, etc. If you are interested, consult a book on SQL.