Use the same technique I suggested before for the plane (mate is also a feature) for editing the dimension directly IDimension::SetSystemValue: https://forum.solidworks.com/message/338855#338855 or use AssemblyDoc::EditMate2. But first method if preferable. If you have a mate name just get it via IAssemblyDoc::FeatureByName.
Application Engineer at Intercad
Tel: +61 2 9454 4444
Thank you very much again Mr. Artem you just helped me again , First I was confused because I'm using C# but now I convert all of it Thank you again