C# code or library to Update virtual directory contents without breaking clients

I have a lot of regularly updating static content that is made available via HTTP through IIS as a Virtual Directory. I have a C# application that needs to update this static content. The static content represents a matching set. This content changes regularly and is validated before being made available to clients. It has a manifest file with version information. I can update the manifest file last but I don't want to pull the rug from under clients that are already downloading older content and leave them with a dirty set of files. What is the recommended way to do a folder replace so that existing clients don't get a mixed up version of the file set? This must be common but I cannot find any libraries or best practice guidance to do this.