We can alter an existing table structure using the ALTER TABLE command, followed by the alteration you want to make:

ALTER TABLE people ADD COLUMN born_year INT;

This will add a new column with empty values:

 age |  name  | born_year 
-----+--------+-----------
  37 | Flavio |          
   8 | Roger  |          

To drop a column:

ALTER TABLE people DROP COLUMN born_year;

This will result in:

 age |  name  
-----+--------
  37 | Flavio
   8 | Roger