I'm not sure if I understand you correctly, but I will try to answer your questions.
First of all, it is not possible to change a modification set offline (in text editor) and upload it again.
If I want to add a user-defined modication set under classic in Administration/Manage Configuration Sets, what should I do?
It is possible only to create a new modification set via "Create" or "Duplicate" in the "Administration".
I know I can click the "creat modification set" to get a new modification set. But I can't change the modification from back end.
What exactly do you mean by "back end"? You can change the modification set in the UI by clicking the "Edit" icon for the modification set you want to change (hover the mouse over the modification set name). Changing the downloaded files (e.g., record.json) within the modification set is not possible.
if I have a new document(record.json and signature Datei) and want to directly upload it under the template classic(System), what should I do?
Otherwise is it possible to change directly change the existing modification set from back end?
If you want to change, e.g., the "configSetRef":"classic" to "configSetRef":"default" or vise versa, this is not possible because the signature file doesn't match anymore.
