On my Elite 5, I can chose between 50khz, 83khz and 200khz, and the downscan works at 455khz.
On my HDS 7 I can chose even more, but I usually leave it on sideimaging most of the time anyways which works at 800khz.
In the menu you chose, sonar> then Frequency, then just change it there.
Surely you can do this on the humminbird also.