Download the appropriate version (32-bit or 64-bit) from Microsoft using one of the links below:

SQL Server 2008 R2 Express - 32-bit Database and Management Tools

SQL Server 2008 R2 Express - 64-bit Database and Management Tools

 Note:  Please ensure that you select the following options during installation:

1) Mixed-Mode Authentication (instead of SQL Server Authentication)
2) SQL Server Management Studion must be installed to allow for manual configuration of the SQL Server instance.