Latest firmware for Duet 0.8.5 is version 1.26.1 which is similar to RRF 2.05.1 for the later Duets. It's available at https://github.com/Duet3D/RepRapFirmware/releases/tag/2.05.1. You should read all the upgrade notes between 1.17 and 2.05.1 before upgrading. In particular, mDNS support was removed and that may affect how you access the web interface.
I understand that F3 made a few firmware changes to RRF 2.x on later boards. I don't know whether they made any to 1.17.