I have built my Thread based product on nRF5 SDK for Thead and Zigbee V2.0.0
What is the best approach to update product firmware to latest stack?
I suggest you read the Release notes in nRF5 SDK for Thread and Zigbee 4.0.0 Introduction, an see what changes have been made since 2.0.0.
I also suggest you also read the release notes of the nRF 5 SDKs that the SDK for Thread and Zigbee is based on, ie nRF5 SDK 15.x.x and 16.0.0, also place in the introduction chapter.