changing partial data

is there a way in postgres to change just a part of a text data in a
column.  for example if i have a column called description with the
following data:

+---------------------+
| description         |
+---------------------+
| this is a test.     |
+---------------------+

and i want to change the word test to exam, can i just replace the word
test rather than reentrying to who value of the column?

so, instead of:

update foo set description='this is a exam' where description='this is a
test';

can i do something like:

update foo replace test with exam;

peter choe