How To Drop UNIQUE CONSTRAINT On Only One Column

I am very new to sql and I have got this doubt.

CREATE TABLE employee
{ EMP-ID VARCHAR UNIQUE,
DESIG VARCHAR UNIQUE,
SALARY INT };

Probably this is worst table ever created :).Now How to drop UNIQUE constraint on only column lets DESIG.

if i write

ALTER TABLE employee
DROP CONSTRAINT myuniqueconstraint;

will drop constraints on both columns which I dont want.

Any query that remove UNIQUE CONSTRAINT only on DESIGN column.

and one more clarification,

ALTER TABLE employee
MODIFY SALARY INT NOT NULL;

ALTER TABLE employee
ALTER COLUMN SALARY INT NOT NULL;

which of the above two is right query to add NOT NULL constraint to the above employee TABLE?