You can't do much with LONGs - in fact, they are more or less obsolete, and new databases should not use LONG (use CLOB if VARCHAR2 isn't big enough).
If the value is under 32K chars you can use PL/SQL:
SELECT longf INTO v_longf FROM table WHERE...;
v_longf := REPLACE( v_longf, ... );
UPDATE table SET longf = v_longf WHERE ...;