Converting prefabs made with an older version

From the menu at the top of Unity, open さたにあ式タバコ (Satania Tabako) → プレハブ更新ツール (Prefab Update Tool).

Put the prefab you want to update into the tool.

Click Prefabを更新 (Update Prefab).

Pick a name and click Save.

If the updated prefab got saved, you're done. Nice work!