download image [[hostname | ipaddress] filename {{vr} vrname}] {primary | secondary} slot slot number
use image {partition} [primary | secondary] slot slotid
reboot slot slot number
The slot number is the one in use by the active node that is to be upgraded. Be sure that you keep the same image versions on all the other nodes as you have on the master node.
Alternatively, if your master node has the same image versions in its partitions that you want installed in the node to be upgraded, you can use the command synchronize {slot slotid} to upgrade both images and select the desired image.