A table contains columns and indexes. Besides, a table has options such as table type, row format and so on. To alter a table means to edit its structure: changing, removing, or adding columns, indexes, modifying table options.
To view or edit structure of a table, right-click it in Database Explorer and select Edit Table from the shortcut menu. Editor tabs appear on the document window. On the Main, Constraints, and Indexes tabs, you can open a dialog to modify item properties. Use right-click to add or remove columns, keys, and indexes. On the Storage tab, you can adjust specific settings for the table. See Defining Table Structure for detailed information.
If you want to undo all actions made since table document (or any another object editor) was opened or saved, click on Undo button on the Standard toolbar. However you cannot issue a Redo command for database objects, because when you click Undo, the database object is reloaded from server, and all unsaved changes are lost.