Before proceeding further check three things.

Your AirPods must be in the charging case. The charging case must be plugged in and charging. An iOS device that’s been connected to your AirPods must be nearby.

If these conditions are met the firmware should automatically update after sometime. But you can also check manually if the firmware is updated or not.

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Make sure your AirPods are paired to your iOS device (opening charging case or putting AirPods in your ears will help you to know pairing status). Launch the Settings app on your iOS device. Go to General. Tap About.

Scroll down and go to AirPods. Check the number next to Firmware Version to see what firmware version your AirPods are running.

3.5.1 is the latest version for your AirPods so if you have this version your AirPods are up to date. If not, you should check internet connectivity on iPhone and you can try again by meeting conditions mentioned in the start of this article.