These are the instructions for SQL 2005, i'm sure it is similar for other versions but you may need to check. Open up the table in the designer and make the changes you want --- but don't apply them. On the top menu you will see "Table Designer" (to the right of file, view, etc) and under that you will see a menu option for "Generate Change Script". Click that and it will open up a window with a query in it. Copy that query to your clipboard, close the window, then close the table designer but DO NOT save. Pop open a new query window, paste in the change script, then take the evening off and wait for it to run.