You use the "Voltage Processor" inverter function.
1. Connect a keyboard output (KEYBOARD CV OUT - X1 or X4) to a "Voltage Processor" input.
2. Switch the relevant "Inverter" On to reverse the signal. (Click on the triangle button (Inverter) between the In B and the Out.)
3. Connect the relevant "Voltage Processor" output to a filter Control input. (That could be into the pre-connected KBD CV input.)
Unfortunately this old ARP2600 V forum does'nt seem to offer a possibility to attach an image or other files. I believe it's possible in the new ARP2600 V3 forum. So if you need an image of an simple example, then post in the new ARP2600 V3 forum about this.)