You can't change the database design? The table you describe... well, it's just a spreadsheet. What advantage do you hope to get from putting it in a relational database?
That said, the UPDATE statement should do what you want. Assuming you're creating records with something like
the following will change ugly_table.february without erasing ugly_table.january:
INSERT INTO ugly_table ( key_1, key_2, january ) VALUES ( 'x', 'y', 200.0 );
Last SQL I did was Informix so I could be misrepresenting the syntax.
WHERE key_1='x' AND key_2='y';