Question to change source code and automatical updates

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Dear users,

I would like to know if It`s possible to change source code that WON`t be overwritten by updating the concrete system?
E.G.: Is there a specific folder where I can put individual source-Code?
Or: What will happen If I e.g. change core-Code?After an update, it will be overwritten, right?
Or do all the updates go in the Update-Folder?

Thank you very much, Kai

KaiTrallafitti