Option	Description
Default instance	If you select Default instance and a default instance already exists on the machine, SQL Server Setup will upgrade the default instance. Choose this option to install a default instance of SQL Server Express. A computer can host only one default instance. Note: The default instance name is MSSQLSERVER; it does not require a client to specify the name of the instance to make a connection. To upgrade the default instance of a previous SQL Server installation, click Default Instance and then click Next.
Named instance	When you run SQL Server Express, by default, the named instance option is selected with SQLExpress as the instance name. Choose this option to create a new named instance. Consider the following when you name an instance of SQL Server: Instance names are not case sensitive. Instance names cannot contain the terms Default, MSSQLServer, or other reserved keywords. If a reserved keyword is used in an instance name, an error will occur. Instance names are limited to 16 characters. The first character in the instance name must be a letter or an underscore (_). Acceptable letters are those defined by the Unicode Standard 2.0, including Latin characters a-z and A-Z, and letter characters from other languages. Subsequent characters can be letters defined by the Unicode Standard 2.0, decimal numbers from Basic Latin or other national scripts, the dollar sign ($), or an underscore (_). Embedded spaces of other special characters are not allowed in instance names, nor are the backslash (\), comma (,), colon (:), semi-colon (;), single quotation mark ('), ampersand (&), number sign (#), or at sign (@). Note: Only characters that are valid in the current Microsoft Windows code page can be used in SQL Server instance names. If an unsupported Unicode character is used, an error will occur.
Installed Instances	Use the Installed Instances page of the SQL Server Installation Wizard to view a list of installed SQL Server Express instances and components on the computer where SQL Server Setup is running. The Installed Instance Details pane displays upgrade options for installed instances. To select a component or instance to upgrade, click the instance name and then click OK.