Vs2017 opens the project and prompts the solution of migration

The possible reason is that we used lower VS Project opened in version , When opening with a higher version , Prompt in the solution that the project needs to be migrated . The solution is ： Open the folder where the project is located , hold Assenbly-CSharp.csproj, Assenbly-CSharp-Editor.csproj, Project name .sln file Delete , Can solve .