The Attach Database Tool is a premium feature, and only available for sites on the Ultimate and Power plan.
The Attach Database Tool is available in Control Panel on the MS SQL Management Page: Home > Sites List > Site Manager > MS SQL Manager > Manage
On the Tools line click the Attach link to get started.
The file that you intend to attach to the SQL database must be an MDF file. Before attaching you must rename your MDF file using the name specified in the MDF File to Attach line in the attach tool. Then upload the MDF file to the App_Data directory of your site.
Any data in the target SQL database will be overwritten when you use the attach tool.