1) Download Target Framework Migrator form the link: https://marketplace.visualstudio.com/items?itemName=PavelSamokha.TargetFrameworkMigrator
(See attached file TargetFrameworkMigrator.vsix).
2) Make sure you close Visual Studio before installing this program.
3) Install the program.
4) After successful installation, open Visual Studio, open the Project(Solution file) that you want to change the Target Framework and then click "Tools" in the Visual Studio Tools menu.
5) Select "Target Framework Migrator" as shown in the below Diagram:
6) As you can see, you can Select All or Some of the projects by selecting the check boxes and then define the Target Framework and then click on "Migrate".
Note: It may take some time depending on how many projects you have in the solution file.