The firmware can be upgraded as long as the current device version starts with the same number. e.g. A device with the version 1.x cannot be upgraded to a version starting with 2.x.
Find also more information on the installation of the software on this page: Firmware
5.1
4.3
4.2
3.5
3.3
3.2
3.1
2.8
2.7
2.6
2.4
2.3