A list of latest firmware version for your Phase One back can be found here
http://www.phaseone.com/en/search/article.aspx?articleid=2161&languageid=1
For IQ: Download the firmware file here:
https://www.phaseone.com/en/SupportMain/Camera-Firmware/IQ
Place it on a CF card and update by going to Menu -> Update firmware on the camera back.
For P and P+ back, download the generic firmware uploader here:
https://www.phaseone.com/en/SupportMain/Camera-Firmware/P
Launch it and connect you back over firewire.
NOTE: Do not use Thunderbolt to Firewire converters!