You can either use the Home app on the iPhone, or manually reset the HomePod on the device itself. Here's how:
Resetting HomePod with iPhone
- Open up the Home app.
- 3D Touch or long press on the HomePod icon.
- Choose "Details" to access your HomePod's settings.
- Scroll down to the bottom of the settings section and tap "Remove Accessory."
- Tap "Remove" again after the confirmation window comes up.
Resetting HomePod Without iPhone
- Unplug your HomePod.
- Plug your HomePod back in again.
- Press and hold the top of the HomePod.
- Continue holding until Siri tells you to let go.
During this process, you will see a red flashing light on the top of the HomePod, and you'll hear Siri say "Your HomePod is about to reset, keep pressing until you hear three beeps." Once those three beeps sound, the HomePod is reset to its original state.
Resetting the HomePod is useful if you run into an issue, but these are also steps that you will need to follow if you want to sell your HomePod or if you need to send it in for service.