Here's a nice little video on how to do it:
But if you don't want to go to Instagram or watch a video, I'll just tell you!
When you turn on your Reverb (or Echosystem), you will see some of the modes lights and presets lights turn on.
The preset light represents the whole number of the firmware version.
Preset 1 BLUE is 1.00
Preset 5 BLUE is 5.00
Preset 1 GREEN is 6.00
The modes light represents the decimal number of the firmware version.
Hall BLUE is 0.01
Sparkle BLUE is 0.05
Hall Green is 0.13
So if your Reverb (or Echosystem) turns on with Preset 2 BLUE and Hall GREEN, the firmware would be 2.13!