Change an existing NOT NULL column to NULL  using alter command.

To change an existing NOT NULL column to allow NULL values, you can use the ALTER TABLE command in SQL. Here are the steps:

1. First, identify the table and the column you want to alter. For example, let's say we have a table named "Employees" and we want to alter the column "LastName" which is currently set to NOT NULL.

2. Use the ALTER TABLE command to modify the column. The syntax would be:

ALTER TABLE table_name
ALTER COLUMN column_name datatype NULL;

3. So, for our example, the command would be:

ALTER TABLE Employees
ALTER COLUMN LastName varchar(255) NULL;

This command will change the "LastName" column in the "Employees" table to allow NULL values. The datatype (in this case, varchar(255)) should be the same as the original datatype of the column.

Please note that the exact syntax might vary slightly depending on the SQL dialect you are using (like MySQL, PostgreSQL, SQL Server, etc.).