i want to have an other menu for my subpages. I created for this a new one.
See this

And now? How can I WP say to add this menu on my subpage instead of the main-menu?

Thanks

Otherwise you will have to create a child theme and modify it a bit to add support for a secondary menu and then assign it to a location in a template file.

The entire process is tut’d here: https://premium.wpmudev.org/blog/add-menus-to-wordpress/

Not a hard as it looks.