Hello,

is there something like a guide for programming a dolphin plugin?

I'd like to change the "overwrite/change name"-window which appears when files shall be copied but exist in the destination folder

Thanks in advance.

Which part of the file name conflict dialog are you looking to customise? Only certain parts of it can be customised using plugins.
The classes you are looking for in this case are KFileMetaDataWidget and PreviewJob. API documentation is available at api.kde.org.