There are two on-chain mechanics available to swap HBD for Hive.
The most common way is the internal market:
https://wallet.hive.blog/market
This market allows us to atomic-swap Hive for HBD and HBD for Hive between users.
The conversion feature is a little weirder and more of an advanced maneuver.
An average for Hive is calculated over 3.5 days and then the HBD is destroyed and creates Hive.