sudo iw dev
or
sudo iwconfig
or
sudo airmon-ng
sudo ip link set <IFACE> down
sudo iw <IFACE> set monitor control
sudo ip link set <IFACE> up
Check again
If you want get back to the Managed Mode
sudo ip link set <IFACE> down
sudo iw <IFACE> set type managed
sudo ip link set <IFACE> up
service NetworkManager restart
sudo ifconfig <IFACE> down
sudo iwconfig <IFACE> mode monitor
sudo ifconfig <IFACE> up
If you want get back to the Managed Mode
sudo ifconfig <IFACE> down
sudo iwconfig <IFACE> mode managed
sudo ifconfig <IFACE> up
service NetworkManager restart
sudo airmon-ng check kill
sudo airmon-ng start <IFACE>
It will create wlan0mon
If you want get back to the Managed Mode
sudo airmon-ng stop <IFACE>
service NetworkManager restart