Update the Firmware in the OnePlus Buds APP
Step 1: Open the OnePlus Buds app and allow the permissions.
Step 2: Go to the Settings section.
Step 3: Make sure to have toggled on the Auto-Update Firmware setting. This will ensure that the earbuds are always up-to-date automatically.
Update the Firmware in the HeyMelody APP
Step 1: Open the HeyMelody app and allow the permissions.
Step 2: Turn on the Bluetooth on the mobile device.
Step 3: Open the charging case and place the earphones inside. Make sure that the earbuds are placed within 15 cm of the device.
Step 4: On the device, tap the “Scan” button to scan the devices nearby.
Step 5: After the scanning is complete, it will show the “OnePlus Buds Z“. If you are unable to find the OPPO Enco Free 2i in the list, then place both the earbuds inside the case and press and hold the multifunction button on the earbuds for 2 seconds to enter the pairing mode.
Step 6: Tap the “OnePlus Buds Z“.
Step 7: Tap “Firmware update” to check for any available updates.
Step 8: If there are updates, tap on them and wait for the time that the application tells you.

Source : thephonetalks.com