Is your monthly cost per apartment stored on your apartment table (the one that stores the apartmentID)? You already have a record for CUSTID 1, APARTMID 1, CONTRDATE 1/1/2013, is the CONTRDATE actually supposed to be 1/1/2014 (as in they signed a new leas or rental agreement)? Are you trying to apply a 15% increase to all properties or just a specific set? how you go about this really depends entirely on your data structure.