Is it possible to change the password for a WiFi network when it is not in range?
You can delete it ('Forget') and create it new again, here you can define the password. Write down the SSID (Routers name) and encryption way (e.g. WPA) before you delete it ;)

Without root you cannot view or edit the /data/misc/wifi/wpa_supplicant.conf file (this is the path on Android CM13), where the network configs are saved. That means ssid, passwords (psk) etc.