https://linuxundich.de/raspberry-pi/drei-wege-das-wlan-auf-einem-raspberry-pi-einzurichten/

update

$ sudo apt update
$ sudo apt full-upgrade
$ sudo reboot

find the wireless adapter

$ iwlist scan
wlan0     No scan results
lo        Interface doesn't support scanning.
eth0      Interface doesn't support scanning.

scan for networks

$ sudo iwlist wlan0 scan | egrep "(ESSID|IEEE)"
  ESSID:"mywireless"
  IE: IEEE 802.11i/WPA2 Version 1
  ESSID:"theothernetwork"
  IE: IEEE 802.11i/WPA2 Version 1

open

$ sudo nano /etc/wpa_supplicant/wpa_supplicant.conf

change

network={
  ssid="mywireless"
  psk="password"
}

restart the wireless adapter

$ sudo ifdown wlan0
$ sudo ifup wlan0

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