This is one of the way to change the build framework in Rider. It allows you to switch the MSBuild version explicitly by selecting MSBuild.dll.
When you have multiple .NET framework, you can change it here.
This example is for .NET core project.
In my system I installed latest .NET core RC after which Rider started complaining in older projects (when there were working fine prior to installation)
I had following versions
I selected the older version of MSBuild.dll and my project were back to normal.