Use /airdrops clear, stop your server, delete the database.db file, then update the plugin.
Read the "Changed" entry down below.
New Features:
Now you can disable airdrop auto-spawning by setting the "delay-treshold" option to 0.
Changed:
All the options in config.yml that where in ticks are now in seconds, this was due to the changes in the spawning and updating logic, ticks based settings are now pointless.
Updated messages.yml placeholders to standardize them (%drop_name%, %drop_id%, %drop_chance% -> %airdrop_name%, %airdrop_id%, %airdrop_chance%), this was due to some placeholders serving the same purpose and being named in a different way.
Added %prefix% placeholder for all the messages in messages.yml so you can decide which ones to put it on.
Improved:
Spawning, updating and removing airdrops performance.