Wifi on NextCloud Box with Raspberry Pi 3

I’ve downloaded the newest Version of UbuntuCore Snap available on NextCloud for Raspberry Pi 3 on my NextCloud-Box.
I installed it and on first boot up with Display and Keyboard set up a wifi-connection. My Question is how to change that Wifi, as I cant write to the wpa_supplicant.conf file in etc. Altough how does the UbuntuCore set up the wifi-connection? As in with which program.
Thanks in advance!