Multiload will only let you update records when the Where clause contains all the primary index fields (and preferably primary key), so you will have to create a file with the primary key of every record to update.
So assuming Table1 has Fields PK1 and Pk2, do a bteq export of:
Select PK1,PK2 From DB.Table1 Where Field1 = 1 ;
The you can import these records to do the update with:
Update DB.TABLE1 SET FIELD1=703 WHERE PK1 = :PK1 And PK2 = :PK2 ;
Make sure there are no updates on the table between the export and import!
If the table has 10M rows, but you only want to update a few thousand, I would go back to your DBA's and argue about bteq. It is not the size of the source, it is how many rows you update which causes the overhead.