Change WiFi config on headless Ubuntu server Pi zero 2 via SD card?

I have a Raspberry pi zero 2 running ubuntu server with a WiFi connection.

I want to take the pi somewhere else and hook onto the WiFi in a new location. I will not know the SSID or the password until I am there. I will have my laptop with me but with no screen, keyboard mouse or ethernet connection for the Pi. Tethering through the laptop is not an option either.

Can I alter files on the SD card to update to the new SSID and password so that the Pi will automatically connect?

If so, which files?

Answering to a one year old question but others may have the same question.

1. before you go, while connected at home:

• sudo cloud-init clean --logs

2. before you go, mount the card in your desktop

• edit /boot/network-config

ethernets:
    eth0:
      optional: true
      dhcp4: true
wifis:  
    wlan0:  
      dhcp4: true  
      optional: true   

or for a new WiFi (I like to use my iPhone hotspot - that way I know the SSID and the password)

ethernets:
  eth0:
    optional: true
    dhcp4: true
wifis:  
  wlan0:  
    dhcp4: true  
    optional: true   
    access-points:  
      "your_SSID":  
        password: "your_netpswd"

3. Before you go, reboot the system twice

• first time copies the new network-config to /etc/netplan/50-cloud-init.yaml
• second time actually uses the /etc/netplan/50-cloud-init.yaml
• if using your iPhone Hotspot, and you can log into the system with ether connection:
  • have the hotspot active when you boot
  • log in from ethernet, run iwconfig to see what the IP address is (it will be the same next time)

4. When you get there

• The Wifi you configured will work (e.g. iPhone hotspot)
• or if you need to set the SSID/password,
  • edit /boot/network-config
  • yank power when green light is off to reboot two- three times

The network configuration files are in /etc/netplan. Change them as you wish :)