Unbound 1.9.2 released (june 17)

you can (have to ?) update the local copy of the root zone using the command:

sudo /usr/sbin/unbound-control auth_zone_transfer "."

I do this on a weekly basis, using cron, also updating the root.hints

You can also increase / decrease the verbose setting using the command:

sudo /usr/sbin/unbound-control verbosity x

x being any number between 0 and ? (I've gone as high as 5)

edit
using the above command eliminates the need to restart unbound, you'll see an entry in the log, if successful.
/edit

In order for this to work, you need to have the following in your unbound.conf file:

# Remote control config section.
remote-control:
	control-enable: yes

Script I run with cron (way to complex, I know):

#!/usr/bin/env bash

# Make sure only root can run our script
if [ "$(id -u)" != "0" ]; then
  echo "This script must be run as root" 1>&2
  exit 1
fi

# compiled version of unbound v1.9.2
if [ -f /etc/unbound/root.hints ]
then
   # unbound
   sudo wget https://www.internic.net/domain/named.root -O /etc/unbound/root.hints
fi
if [ -f /etc/unbound/root.zone ]
then
   # zone transfert
   echo 'zone transfert "."'
   sudo /usr/sbin/unbound-control auth_zone_transfer "."
fi

# Raspbian version of unbound v1.6.0
if [ -f /var/lib/unbound/root.hints ]
then
   # unbound
   sudo wget https://www.internic.net/domain/named.root -O /var/lib/unbound/root.hints
fi