To alter a column, select "Alter" from the "Select" list. Choose "Alter" from the Alter list. You will have two options at this point : Set and Drop.
To set the default value for a column, select the column that you want to set the default value for and enter the default value in the "Column Name" textbox and click the "Build SQL Query" button. After reviewing the SQL query click the "Run SQL Query" button to complete the operation.
To drop the existing default value of a column, choose the "Drop" option. Select the column you want to drop the default value for and click on the "Build SQL Query" button. After reviewing the SQL query click the "Run SQL Query" to complete the operation. The result of the operation will be displayed at the bottom of the page.