I updated my vs 2019 16.3.10 to 16.4 and .net core 3.0 to .net core 3.1 but the blazor web assembly application template is missing.
In command line run:
dotnet new -i Microsoft.AspNetCore.Blazor.Templates::3.1.0-preview2.19528.8
cd C:\Users\Username\Documents\MyWebSites\BlazorWebAssySite1
dotnet new blazorwasm
Open VS2019, choose open project and find BlazorWebAssySite1 folder. Done. For new projects just create a new project folder and run the last command.
Have also a look at this video: https://www.youtube.com/watch?v=bophIr--1QY .