There are a handful of third-party C# libraries that can do this, such as OfficeWriter (who I work for) and others.
Using the library and your Word document you simply call a method, passing in the data to be mail merged. The API is usually pretty strait forward:
using (WordTemplate wordTemplate = new WordTemplate())
{
wordTemplate.Open("YourFilePath.docx");
// yourData is a a DataTable in this case,
// but SetDataSource has many overloads
wordTemplate.SetDataSource(yourData);
wordTemplate.Process();
wordTemplate.Save("OutputFilePath.docx");
}