Existing DB VM homes can only be updated with the same major release. For example, you can update an Oracle DB home from 12c April 2018 Proactive patch to 12c October 2018 Proactive patch. You cannot upgrade an existing DB home from one major release to another.
However, you can install the Oracle Database of your choice (12.2c and 18c, for example) and then create new DB homes and instances for the DB VMs if you follow these guidelines:
Ensure that the Grid infrastructure and Shared filesystem components are running Grid Infrastructure 18c. Use these procedures to view, and if needed, upgrade those components:
Create a new DB home. See Create DB Homes (BUI).
Create DB instances of the new Oracle Database release. See Create DB Instances (BUI)