With the release of Windows 8 Final, Microsoft has also released Visual Studio 2012 and .NET framework 4.5 to the web. Like Windows 8, Visual Studio 2012 and .NET Framework has been made available to MSDN and TechNet subscribers first and will be available to others afterwards.
According to Microsoft:
The Microsoft .NET Framework 4.5 is a highly compatible, in-place update to the .NET Framework 4. By using the .NET Framework 4.5 together with the C# or Visual Basic programming languages, you can write Windows Metro style apps. The .NET Framework 4.5 delivers better performance, reliability, and security.
I have already written about .NET Framework 4.5 developer preview which was highly compatible with the previous versions of .NET Framework like 3.5 and 4.0. It could be installed along side those Framework installations. The final version of .NET Framework 4.5 also maintains the same compatibility and can be installed along with the previous installations of .NET Frameworks.
Since .NET Framework 4.5 is already a part of Windows 8, you will only need to update Windows 8 in order to update .NET Framework 4.5 installation.
System Requirements for .NET Framework 4.5
Supported Operating Systems
Windows Vista SP2 (x86 and x64)
Windows 7 SP1 (x86 and x64)
Windows 8 (x86 and x64)
Windows Server 2008 R2 SP1 (x64)
Windows Server 2008 SP2 (x64)
Windows Server 2012 (x86 and x64)
1 GHz or faster processor
512 MB of RAM
850 MB of available hard disk space (x86)
2 GB of available hard disk space (x64)
350 MB of available hard disk space (language pack)