Backup from the web UI is no longer available in latest version. As you have already taken a snapshot, I would recommend you upgrade to the latest Central version.
How do I backup the REDIS for Enketo
You'll need to dump the Enketo databases and secrets from your server by running the following commands:
docker exec central-enketo_redis_main-1 redis-cli -p 6379 save;
docker cp central-enketo_redis_main-1:/data/enketo-main.rdb ~/enketo-main.rdb;
docker exec central-enketo_redis_cache-1 redis-cli -p 6380 save;
docker cp central-enketo_redis_cache-1:/data/enketo-cache.rdb ~/enketo-cache.rdb;
docker cp central-service-1:/etc/secrets ~/;