In Visual Studio 2019.
Open your web forms project.
go to References (in solution explorer).
Right click references...
Select Manage Nuget Packages...
You will see something like the following:
It shows Bootstrap 3.4.1.
You will have to select version 4.5.3 of bootstrap on the right side you can select [Update] button.
If you try 4.6.1 it won't work.