Currently there is no easy I am aware of how you can completely empty or delete a large amount of white/blacklist items after the migration to 5.0. I would like to see a button to completely empty those lists and maybe check boxes to select multiple entries or all currently shown entries.
You go the mentioned page and export you settings. This your backup and lifesaver.
Run in the CLI
sudo pihole -g -r erases your lists and adlists
Again to the page and mark the settings/lists you want back.