Your VPS comes with full access to its operating system, so you manage its firewall from inside the server. On Ubuntu and Debian the easiest tool is UFW:
sudo apt install ufw sudo ufw allow OpenSSH # always allow SSH first, or you'll lock yourself out sudo ufw allow 80/tcp # web (HTTP), if needed sudo ufw allow 443/tcp # web (HTTPS), if needed sudo ufw enable sudo ufw status verbose
Locked yourself out? Log in through the noVNC console in the Client Area and run sudo ufw disable.
Need firewall rules applied at our network level instead? Open a support ticket.