also how do i delete all the current exterior navmesh's?
does it overwrite when i generate?

Would be better to edit the existing ones so as to not isolate the NAVM forms. If another plugin's NAVMs/NAVI references deleted [00] NavMeshes, nothing good will become of it.

Note: Before editing a Skyrim.ESM NAVM, give it an EditorID (its FormID works well) to ensure you don't inadvertently replace it.