Hi,
You can simply use triggerhappy as described here:
Basically all you have to do is to edit the file (here using nano editor) - of course you should access Volumio via ssh. The edit means you should remap action assigned to particular key of your remote.
file:
sudo nano /etc/triggerhappy/triggers.d/audio.conf
After file edit, to make effect of changes a system reboot or triggerhappy process restart is needed:
sudo systemctl restart triggerhappy.service
Ussing triggerhappy you can check what is the exact kay name to be remapped using logging command:
cd /usr/sbin
sudo ./thd --dump /dev/input/event*
Good luck