This may be useful when for example the character encoding has been messed up and you need to correct existing data. This example will correct swedish accented characters in a uft8 - latin1 mixup.
UPDATE my_table
set my_field = replace(my_field,'Ã¥','å');
UPDATE my_table
set my_field = replace(my_field,'ä','ä');
UPDATE my_table
set my_field = replace(my_field,'ö','ö');
UPDATE my_table
set my_field = replace(my_field,'Ã…','Å');
UPDATE my_table
set my_field = replace(my_field,'Ä','Ä');
UPDATE my_table
set my_field = replace(my_field,'Ö','Ö');
(Yes, you can concatenate them if you like but they become very hard to read.)