UPDATE [tablename] SET [fieldname] = REPLACE([fieldname],"[oldstring]","[newstring]");
The code above would operate on every row in the table but you may want WHERE conditions. Example:
UPDATE profile SET homepage = REPLACE(homepage,"blargh.example.com","foo.example.com") WHERE first_name="Joe" AND last_name="Schmoe";
Will change Joe's homepage to http://foo.example.com/ instead of http://blargh.example.com/.