Thats right @liamh. It will be easier to navigate with a graphical file manager with the mouse to the root directory of the card where you will see the “etc” folder and right click in a blank space and choose “command line”.
From there choose
sudo nano ./etc/wpa_supplicant/wpa_supplicant.conf
You will be prompted for administrator’s password of your Ubuntu machine.
Once the editor is up, use only the keyboard (mouse does not move the cursor) and change your wifi details. When everything is correct press Ctrl+O and then ENTER to save. And finally Ctrl+X to close the editor.
Lastly unmount the SD card from your Ubuntu machine and insert it in HestiaPi.
Let us know if you made it!