For the sake of completeness and the edge case of wanting to update all columns of a row, you can do the following, but consider that the number and types of the fields must match.
Using a data structure
exec sql UPDATE TESTFILE
SET ROW = :DataDs
WHERE CURRENT OF CURSOR; //If using a cursor for update
Source: rpgpgm.com
SQL only
UPDATE t1 SET ROW = (SELECT *
FROM t2
WHERE t2.c3 = t1.c3)
Source: ibm.com