Where do I set/change my Google Chrome passphrase?

The passphrase will be your Google account password. If you've changed your Google account password after setting up sync & encryption, the passphrase will still be the old password.

You can reset your Google Sync passphrase at your Google Dashboard.

More information see the support article Protect your synced data:

If you forget your passphrase, you’ll need to reset sync via the Google Dashboard. This will delete all synced data from Google’s servers and disconnect all synced computers and devices, but not the data that’s on your computers or devices. So your current preferences, bookmarks, and passwords will remain available in the browser. You can then re-enable sync with a new passphrase.

I couldn't find proper answer to this questions after 45 minutes of Googling. Here is what you need: https://support.google.com/chrome/answer/1181420?hl=en

Don't be afraid of clicking STOP and CLEAR button, it won't delete any data on your local computer. When you re-sync on Chrome, using your Gmail password as your pass phrase.

Good luck!