There are two options:
Restore a backup from before you made this mistake (Backups - Anki Manual).
Sync your iPhone to AnkiWeb and force download your AnkiWeb state to your desktop (Syncing with AnkiWeb - Anki Manual):
If you wish to force a full upload or download (for example, because you accidentally deleted a deck on one side and want to restore the deck rather than having its deletion synchronized), you can check the “On next sync, force changes in one direction” box in Tools>Preferences>Network, then sync as usual. (You’ll be given the option to choose which side you want to use.)