You can use the UPDATE statement to change the column values of existing rows.
For example, the following example shows how to offer anyone who is a manager a 10% raise:
SELECT ename, sal FROM emp WHERE job = 'MANAGER';
ename | sal
-------+---------
JONES | 2975.00
BLAKE | 2850.00
CLARK | 2450.00
(3 rows)
UPDATE emp SET sal = sal * 1.1 WHERE job = 'MANAGER';
SELECT ename, sal FROM emp WHERE job = 'MANAGER';
ename | sal
-------+---------
JONES | 3272.50
BLAKE | 3135.00
CLARK | 2695.00
(3 rows)