Best way to update LINQ to SQL classes after database schema change

You can use SQLMetal.exe to generate your dbml and or cs/vb file. Use a pre-build script to start it and target the directory where your datacontext project belongs.

C:\Program Files\Microsoft SDKs\Windows\v6.0A\Bin\x64\sqlmetal.exe 
  /server:<SERVER> 
  /database:<database> 
  /code:"path\Solution\DataContextProject\dbContext.cs" 
  /language:csharp 
  /namespace:<your namespace>

I haven't tried it myself, but Huagati DBML/EDMX Tools is recommended by other people.

Huagati DBML/EDMX Tools is an add-in for Visual Studio that adds functionality to the Linq2SQL/DBML diagram designer in Visual Studio 2008, and to the ADO.NET Entity Framework designer in Visual Studio 2008 SP1. The add-in adds new menu options for updating Linq2SQL designer diagrams with database changes, for renaming Linq-to-SQL (DBML) and EF (EDMX) classes and properties to use .net naming conventions, and for adding documentation/descriptions to Linq-to-SQL generated classes from the database properties.