Open your project in VS. Go to the Qt menu (on the menu bar in VS - it is added when you install the Qt VS addin). Select the "Qt Project Settings" command. When the dialog appears, select the "Properties" tab. Click "Version" and from the drop-down combo box select either your current Qt version or "$(DefaultQtVersion)". Click OK to close the dialog, rebuild

Alternatively, right click on the project name in the Solution Explorer, select "Qt Project Settings" to get you to the same dialog.