Set for multiple fields in update query

Yes, upsert doesn’t support FROM.
My full raw SQL query looks like:

INSERT INTO 
    table (field_a, field_b)
VALUES 
    ('', ''),
    ('', ''),
    ('', '')
ON CONFLICT (field_a, field_b) 
DO 
  UPDATE SET (field_a, field_b) = (select * from my_function(field_a, field_b, $1, $2) as (value1 uuid, value2 timestamp));