I'm working with a server that I don't want to connect it to Internet because of it's importance. How can I update openvas from connecting to another local server?
you need to have a “proxy” server (either full GVM installation, or just a rsync-client) that gets updates from community feed. Then you can distribute updates to your local network by enabling rsync-server in “proxy” for more details: https://community.greenbone.net/t/nvt-sync-a-security-risk/693/6