I have Windows 10 x64 with installed latest Visual Studio 2015. I needed v141 build tools so I downloaded and installed Visual Studio 2017 C++ Build tools. Anyway the the IDE still showing only v140 in Platform Toolset listbox in project settings.
I checked if it's installed and it's contains core tools, mstools and tools v141.
In case you downloaded a project that wrote in VS2017, and your VS is 2015.
You don't need to buy a new version of VS, just open solution explorer and right click on properties and then select general from left panel and change the platform toolset form V141 to V140.
Most projects which build in VS2017 don't necessarily use new features in its V141 toolset.
