Three easy steps:

Here it goes

root@kali:~# ifconfig wlan1 down
root@kali:~# iw reg set BO
root@kali:~# iwconfig wlan1 txpower 30

This is what it looks before

wlan1 IEEE 802.11bg ESSID: off/any
Mode:Managed Access Point: Not-Associated Tx-Power=20 dBm
Retry long limit:7 RTS thr: off Fragment thr: off
Encryption key: off
Power Management: off

This is what it looks after

after :
wlan1 IEEE 802.11bg ESSID: off/any
Mode:Managed Access Point: Not-Associated Tx-Power=30 dBm
Retry long limit:7 RTS thr: off Fragment thr: off
Encryption key: off
Power Management: off

mine is wlan1 as internal is wlan0 yours may well be wlan0 so just swap wlan1````for ````wlan0 in your own setup.