We highly recommend you to do a backup of your instance before doing any of the mentioned steps for migrating or updating CouchDB to ensure you won't lose your data. You can get more information about how to backup a server in Google Cloud Platform in the link below
According to the CouchDB wiki you can simply copy the files from one instance to another. In the
/opt/bitnami/couchdb/etc/local.ini config file you can see the location of the database and plugins files
Find below a guide about how to upgrade and migrate CouchDB from the official CouchDB docs
Please note that we generate all the binaries present in the Bitnami CouchDB stack to ensure that they are properly configured and that they will work properly into the Bitnami Stack environment. Because of that, you could not upgrade the CouchDB version present on your current instance and you will need to migrate your data to a new one. Please, remember to do a backup of your instance before starting the migration process so you can get back to the previous state and restore your information in case you find any issue.
Let us know any other question you may have.