If I remember correctly, the battery on some of the Panel Mate models is not considered replaceable. I'm not sure if that applies to the 1700 series. That means there is no factory procedure to replace them nor is the battery identified in the spare parts list. I think they expected the battery to outlive the HMI. You will have to disassemble the unit, locate the battery and find a replacement for it. It's probably soldered onto the mother board. Either that or replace the PM1700.
You might want to try to download a simple configuration that doesn't use the PM1700's internal clock to see if battery failure is fatal if you don't need the RTC.