Lumia 640 and Lumia 640 XL are eligible to receive the Lumia Denim update. Here is a step by step guide to get your hands on it:

Lumia Denim, WiFi Calling, Volte Rolling Out To Microsoft Lumia 640, Lumia 640 XLLumia 640 owners with subscription to EE UK or T-Mobile US will be eligible for native Wi-Fi calling functionality post-update. They can also access VoLTE compatibility active on specific wireless carriers.

Also, on cards are audio call enhancement and add-ons to cellular connectivity, stability for network and national roaming. The latest software version for Lumia 640 is 02177.00000.15184.36xxx/8.10.15148.160, says Microsoft.

As for users of the Lumia 640 XL, they are also eligible to get hands on VoLTE support via selected operators besides native Wi-Fi calling support if subscribed to EE in the UK. The update also brings in enhancements to audio call quality, Wi-Fi, connectivity, roaming and network stability, again. Lumia 640 XL comes with the version 02177.00000.15184.36xxx/8.10.15148.160 for the update.

Simply select the menu in this order– Settings Phone Update Check for Updates to access the latest update. In case, you are yet to get the same, it just might be due to your region and location.