In Visual Studio 2008, on the File menu, point to New and then click Project.

Doing this displays the New Project dialog box.

Select a project type, such as Visual C# Windows Application.

In the upper-right corner of the New Project dialog box, click the combo box and select the .NET Framework version you want your project to target, for example, .NET Framework 3.5.

Click OK and then continue to create your project as usual.

In Visual Studio 2008, open the project you want to change.

Right-click the project in Solution Explorer and then click Properties.

Doing this displays the Project Designer.

Go to the Target Framework drop-down list.

For Visual Basic projects, click the Compile tab and then click Advanced Compile Options. The Target Framework list is in the Advanced Compiler Settings Dialog Box (Visual Basic).

For Visual C# projects, the Target Framework list is on the Application tab of the Project Designer. For more information, see Application Page, Project Designer (C#).

In the Target Framework list, click a .NET Framework version that differs from the current one for the project.

The project now targets the new .NET Framework version. The project will unload and then reload in the integrated development environment (IDE). 

Note:
If you change the .NET Framework target version, you may receive error messages if your code contains references to a different version. To resolve these errors, you must manually modify the references.

In Visual Studio 2008, open the project that you want to target .NET Framework 3.5 SP1.

Perform one of the following actions that enable an SP1 ClickOnce feature:

• Specify an Error URL in the Publish Options dialog box.

• Specify a Suite name in the Publish Options dialog box.

• Create a desktop shortcut in the Publish Options dialog box.

• Exclude a file from the hash in the Application Files dialog box.

• Clear the Sign the ClickOnce manifests check box on the Signing page.

-or-

• Add a reference to the System.Data.Entity assembly.

Note:
If you change the .NET Framework target, you may receive error messages if your code contains references to a different version. To resolve these errors, you must manually modify the references.

In Visual Studio 2008, open the project you want to change.

Right-click the project in Solution Explorer and then click Properties.

Doing this displays the Project Designer.

Click the Compile page.

Click Advanced Compile Options.

Select Client-only Framework subset.

In Visual Studio 2008, open the project you want to change.

Right-click the project in Solution Explorer and then click Properties.

Doing this displays the Project Designer.

Click the Application page.

Select Client-only Framework subset.