REPLACE(str,from_str,to_str)

Returns the string str with all occurrences of the string from_str replaced by the string to_str.

 examples :

• replace string in all records from a table :
   update mytable set myfield =  replace (myfield, "one", "two");
• display a modified record set
  SELECT REPLACE(myfield, 'one', 'two') from mytable;

more info about string functions on mysql .com, be careful if you're using version 3, some functions appeared with version 4 

http://dev.mysql.com/doc/mysql/en/string-functions.html