Hello Jay,
unfortunately you can use a scalar UDF only in the SELECT or WHERE clause of a SELECT statement, but not directly in an UPDATE statement.
You can transform your UPDATE statement to use a subselect to be able to use your UDF. Something like following:
UPDATE MYTABLE as "T1"
SET "REFORMATTED_STRING" = ( SELECT FCN_REFORMAT_STRING("ORIGINAL_STRING") as "REFORMATTED_STRING" FROM MYTABLE as "T2" WHERE "T1".<key> = "T2".<key>");
Of course you can do your reformat also within a procedure or depending on the complexity of your reformat directly with the UPDATE statement using expression.
Best regards,
Florian