The components of the .NET Framework are considered a core Windows feature, therefore there are no facilities to reinstall them. However, there are tools to verify and repair system files.
- Run SFC Scannow - https://support.microsoft.com/en-us/kb/929833
- Run .NET Framework repair tool - https://www.microsoft.com/en-ca/download/details.aspx?id=30135
Running those tools may take a long time.