Code:
SQL> select * from xyz;
X
--------------------
lasjkld.abc
jkaasjhd.abcsjdjjsj
u.abc.jaslkdjsysyy
.abcsjdjshdysy.ksjs
SQL> select x, replace(x, '.abc','.defg') newval
2 from xyz;
X NEWVAL
-------------------- --------------------
lasjkld.abc lasjkld.defg
jkaasjhd.abcsjdjjsj jkaasjhd.defgsjdjjsj
u.abc.jaslkdjsysyy u.defg.jaslkdjsysyy
.abcsjdjshdysy.ksjs .defgsjdjshdysy.ksjs
SQL> update xyz set x= replace(x, '.abc','.defg');
4 rows updated.
SQL> commit;
Commit complete.
SQL> select x from xyz;
X
--------------------
lasjkld.defg
jkaasjhd.defgsjdjjsj
u.defg.jaslkdjsysyy
.defgsjdjshdysy.ksjs