In the QBE grid, you can do the following:
Set up an UPDATE query. Select field1 as the field to be updated. In the Criteria box, enter the following formula:
IIf([field2] = "that","this","thatotherthing")
If you need to test field2 for more than one value, use a nested IIf (see the Help file for assistance).
If you want to do this in code, open a recordset based on the table. Assign field1 and field2 to variables, then use a SELECT CASE statement to test the value of field2 and update field1 accordingly.
Hope it helps,
j-Dog