The Schema Viewer offers the ability to add, edit, or delete the columns in a table. Because different database servers differ widely in what features they offer for modifying existing tables, some or all of these features might not be available with the database you are connected to.
To add or delete a column, select select the appropriate item from the Schema menu.
To edit a column, select a column in either the fourth column of the schema browser or from the list of columns displayed at the bottom of the window when Columns is selected in the schema browser. Then select Edit Column... from the schema menu.
The interface for adding or editing a column is identical, except the fields will be filled in with the current values if you are editing a column. Also, some options available for new columns might not be adjustable for existing columns.
For a detailed description of the fields available, see the documentation on creating a table.