the cloudeos images are upgraded/downgraded just like any other arista switch by copying the desired cloudeos.swi file to /mnt/flash. Configure the boot system flash:cloudeos.swi, and then reload the VM from the arista CLI. For more information, refer https://www.arista.com/en/um-eos/eos-upgrades-and-downgrades. Note, while upgrading your existing router image to cloudeos router image, use .swi as the file extension.
the appliance itself can be upgraded apart from the cloudeos VMs. Refer to Appendix E - Tools to Manage and Update Images section in arista cloudeos and veos router appliance Guide guide for steps to upgrade CVA. During CVA upgrade process, all dca-200-veos scripts under /data/imaging/ and /data/tool/ directories are also upgraded. there will be a newly created directory named as current CVA version under both /data/imaging/ and /data/tool/ (For example, /data/imaging/2.1.2/ and /data/imaging/2.1.2/). Older version of the scripts are moved down to that version directory. Newer version of the scripts are copied to /data/imaging/ and /data/tool/ directly.
After a system reboot from the last step of upgrade, the cloudeos instances becomes up and runs automatically after appliance is up again. Allow 20 minutes for the application running in cloudeos instances to be accessible again.