If you're referring to a problem at runtime after swapping versions of your assembly without performing a rebuild of the program referencing your newly built assembly, you'll want to use a <bindingRedirect> directive to your program's App.config (or Web.config, if you're talking about a web site.)
bindingRedirect is used to instruct the .NET Framework that it's OK to use a version of an assembly other than the one the application was originally compiled against. By default, the CLR wants to see the same version of a DLL that your application was referencing during build, and if it doesn't it will throw an exception.