Did you just copy your initial post by accident? I'm asking for more information because I'm not going to be able to help you with the little info you're providing.
I’m still not understanding exactly what you are trying to do. Do you just mean you want to sync your desktop library with the online library at Zotero.org? If so, just set up Sync in the Zotero desktop preferences.
If you mean something else, can you please describe in much more detail exactly what situation you are in and what you are trying to do. Please write several sentences to explain your situation completely. If a language other than English is easier, please feel free to use that.
I want to add a new database to Zotero and sync the web version from there. But first you need to completely delete the old database from the Zotero program. It won't work. The data (bibliographic records) still remain somewhere. How to delete the database completely?
You can either sync to a new account -- that automatically empties the local database (you'll get a warning to that extent) -- or you can move all items in your local library to the trash, empty the trash, then sync, that will completely (and irretrievably, so careful!) empty your local database.
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If you mean something else, can you please describe in much more detail exactly what situation you are in and what you are trying to do. Please write several sentences to explain your situation completely. If a language other than English is easier, please feel free to use that.