OK I have sorted the partition issue thanks to UniPi tech support: manually resizing the SD card partition using gparted on another Linux system.
However I'm still looking for a guide to the 'official' way to upgrade nodeJS, npm and NodeRed on a UniPi Neuron. I'd like nodeJS 18 and Node-RED 3 if possible. The only documentation I came across regarded nodeJS 14 as 'new'!