Fast answer : You can't!
Long answer : I don't think someone would spend time on a firmware for a discontinued product. So, you can't!
Is it that hard to divide the depth by around 3 (in your head)to have your measure?
The way I work, for dual units, is that I have a Garmin GPS connected to my sounder.
And on the Garmin, you can select the data you want. So I have imperial units in the HB and metric Units in the Garmin. Best of both worlds. (Since I use both of them)