If your website is using a Theme developer by somebody else, there is the chance that your content will "disappear" after you activate the newly installed Theme. No worries, the content is not lost, if you do reactivate the previous Theme, the content will still be there.
This happens because each Theme defines the types of content differently. These steps aren't normally necessary on Posts (Beiträge) and Pages (Seiten), as those are standard WordPress content types.
There are different options to proceed with the migration of those content types and we'll list them here:
Exporting/importing the content with a Plug-in (do it yourself, at your own risk)
Export Plug-in:
Import Plug-in:
Here the guides to Export/Import without WPML enabled:
https://www.wpallimport.com/documentation/export/overview/
https://www.wpallimport.com/documentation/getting-started/wp-all-import-in-depth-overview/
If your website is supposed to be Multilingual, before starting the import of the content you should set up WPML and then also install this support Plug-in for the import using WPML: