Upgrade Ubuntu 18.04 LTS to 20.04 LTS with Zimbra 9 build by Zextras

Hi Everyone, hope this is the right place to ask the question  below.

I have a Ubuntu 18.04 VM on proxmox hipervisor on which is running Zimbra 9 build by Zextras. Everything is working fine. Now I need to update Ubuntu to 20.04 LTS. Could someone tell me if there is a good howto in order to update Ubuntu and Zimbra 9 build by Zextras ? On the same server I have also installed a let's encrypt certificate. Thank you in advance for the help.