Upgrade in Steps for NetoniX Switches

[DFC001]

I need to upgrade from firmwares 1.5.2 & 1.5.4. I'd Like to match what we have 1.15.14 / Q1. Do I need to upgrade in Steps, if so what are the steps? / Q2. for the older firmware I can't upgrade via GUI, what is the best way to upgrade without bricking the switch? / The Models are> WS-10-250-AC & WS-12-250-AC