First go into the Settings/Information/System information tab in the Prisma app and confirm that the update has not already taken place by confirming the firmware versions on your Prisma device with information found in the Current Firmware tab in this FAQ.
If the update has not taken place you can force an update by rebooting the Prisma device, by either unplugging it from power and then reconnecting the power supply if an NP5 Prisma or by turning off and then back on using the AC mains power switch on the back panel.
This will cause the Prisma device to do a full systems check, including checking for any new firmware updates, which will be installed if any are available.
Do allow a good ten minutes (it should not take this long, but just to be on the safe side) for this full process to take place, being sure not to use it during this process so that it can be completed without interruption.
If for some reason it is interrupted and gets stuck, simply reboot again.