I change it like this , no need for reboot and "dpkg-reconfigure"
Editing dump1090-mutability configuration
look for Gain setting section and change itCode:
sudo nano /etc/default/dump1090-mutability
# RTLSDR gain in dB.
# If set to "max" (the default) the maximum supported gain is used.
# If set to "agc", the tuner AGC is used to set the gain.
Save file CTRL+O
Enter and exit CRTL+X
sudo /etc/init.d/dump1090-mutability force-reload
sudo /etc/init.d/dump1090-mutability restart
[ ok ] Restarting dump1090-mutability daemon: dump1090-mutability.
Thanks, your's is easier method than what I did.