To unblock an IP address that you have previously blocked, you can use the same tools used for blocking. If you have used iptables, you can remove the block rule using the 'sudo iptables -D INPUT -s -j DROP' command, where is the IP address you want to unblock. If you have used ufw, you can delete the deny rule using the 'sudo ufw delete deny from ' command. If you have used fail2ban, you can unban an IP address using the 'sudo fail2ban-client set unbanip ' command, where is the name of the fail2ban jail and is the IP address you want to unblock. Remember to verify that you are unblocking the correct IP address to avoid any unintended consequences.