Modifying text data?

Hi,
 I'm trying to modify text fields in an "automatic" way so to be able
for example, to change any 'Canada' substring occurrence with the
UPPER CASE equivalent "CANADA", with the added option 'Canada'
appearing anywhere inside the text field.
While it is very easy to find matching registers by using regular
expresions, (SELECT * from my_table where my_columm ~* 'canada'; in
previous example) I find no way to change them afterwards, but
manually updating each register.

 Thanks in advance for any suggestions!