how we can modify execute table fields in SQL server 2008 express database?

how we can modify execute table fields in SQL server 2008 express database?

The problem is , when i add field or modify the datatype value for
executed table it shows the error that database table can't modify.
Plz solve my problem. i face to much problem to add or to change somthing
in field of any table.
I had intalled SQL Server 2008 Express edition.