Hey @pdsavas,
You can insert the SD Card in your computer and a resin-boot partition will open.
Inside the partition, you will find a folder called system-connections where there is a sample file with .ignore extension. Copy that, remove the .ignore from the filename, and edit to add your wifi credentials.
Alternatively, if you connect to the device over ethernet, log into the host OS, then change the file in /etc/NetworkManager/system-connections. Change the example file mentioned above with a text editor such as vim and then restart the device.
This should do it 