There may come a time when you need to retrain your Google Assistant's voice model---that is, the one that detects the "OK Google" command. For example, "Hey Google" recently started rolling out on phones, so you'll need to retrain the voice model to accept this new phrase.
It's worth mentioning that retraining your voice model doesn't just apply to the device you retrain. When you retrain your device, it applies to all of your devices that use Google Assistant. And with that, let's do this thing.
First, launch Assistant by whatever means you normally would: long-press the home button, say "OK Google," etc. When it's open, tap the little tray icon in the upper right corner.
When the "Explore" menu opens, tap the three dots in the upper right corner, and then choose the "Settings" option.
On the "Settings" page, select the "Voice Match" option.
To retrain your voice model, tap the "Teach Your Assistant Your Voice Again" option.
A dialog box pops up asking if you want to retrain the voice model, noting that it will apply to all speakers. Tap "Retrain" to continue.
Follow the instructions, repeating "OK Google" and "Hey Google" (if this is available on your account). Once finished, the new voice model uploads to your account, and you're done.
Easy peasy.
Like I said earlier, "Hey Google" isn't yet available to all users, but it's currently rolling out on a per-account basis. If you don't have it yet, you should soon.