Let's start with a simple example of a table, as follows:

postgres=# select * from birthday;

This gives the following output:

 name  |  dob   
-------+--------
 simon | 690926

(1 row)

The preceding table was created using this command:

CREATE TABLE birthday
( name       TEXT
, dob        INTEGER);

Let's say we want to change the dob column to another data type. Let's try with a simple example first, as follows:

postgres=# ALTER TABLE birthday
postgres-# ALTER COLUMN dob SET DATA TYPE text;
ALTER TABLE

This works fine. Let's just change that back to the integer type so that we can try something more complex, such as a date data type:

postgres=# ALTER TABLE birthday
postgres-# ALTER COLUMN dob SET DATA TYPE integer;
ERROR:  column ...