There's a neat project on The Code Project that will issue updates for the browserCaps section of the machine.config file which is located in %windir%\Microsoft.NET\Framework\<version number>\CONFIG. Note that this applies only to ASP.NET web sites, IIS has another file (%windir%\system32\inetsrv\browscap.ini) that will be used for non-ASP.NET web sites.
BrowserCaps are used to determine which browser type visits a web site. ASP.NET will decide upon this information how to render content for specific browsers, versions and platforms.
There's also a write-up on how to use various ASP.NET configuration files and settings.