Use wget command to retrieve file and store to /etc/bind/db.root (Debian / Ubuntu Linux), enter:
# wget --user=ftp --password=ftp ftp://ftp.rs.internic.net/domain/db.cache -O /etc/bind/db.root

Under Red Hat / CentOS / Fedora Linux, default location is /var/named/named.root, enter:
# wget --user=ftp --password=ftp ftp://ftp.rs.internic.net/domain/db.cache -O /var/named/named.root

Reload rndc to update information, enter:
# rndc reload

Another option is run dig command to fetch information:
# dig +bufsize=1200 +norec NS . @a.root-servers.net > /var/named/named.root