Extract, edit and recompress. Paths are for the class template, but the interface templates are in the same folder.
You may want to edit the VS template file in each to remove the fact that they don’t automatically add references to the assemblies System, System.Data and/or System.Xml.
2005:
C:\Program Files (x86)\Microsoft Visual Studio 8\Common7\IDE\ItemTemplates\CSharp\1033\Class.zip
2008:
C:\Program Files (x86)\Microsoft Visual Studio 9.0\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class.zip
2010:
C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class.zip
Starting with Visual Studio 2012, the templates are not zipped, so you can edit each .cs template in each applicable folder directly.
2012:
C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class\Class.cs
2013:
C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class\Class.cs
2015:
C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class\Class.cs
2017
VS 2017 changes the directory location, and is now dependent on your edition (Professional/Enterprise/etc). So for the Enterprise edition:
C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class\Class.cs
2019
The VS 2019 location is similar to 2017. So for the Enterprise edition:
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class\Class.cs
Notes
Express Editions
In Express Editions you will have to search in the subdirectory WDExpress inside the IDE folder, so e.g. for VS 2015 Express:
C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\WDExpress\ItemTemplates\CSharp\Code\1033\Class\Class.cs
Other Languages
If you don’t use the English version of Visual Studio, the folder 1033 may not exist, but a different number representing your language. For example it is 1031 in a German installation.