Right-click your project and click in “Properties”. In the tab “Application” see if you have the option for the 4.5 framework in the “Target Framework” dropdown list. If you have it, just select it and try to build your project again.
If you don’t have this option, go to “Programs and Features” in Windows 7 and make sure you have .NET Framework 4.5 installed. If you have it, then try repairing your installation of VS 2012 in this same menu and verify again your options in the “Target Framework” dropdown list.