One way to accomplish this is from the Keygroup Mapper (the part of the interface with the keyboard):

Each sample will be shown as a vertical bar above its respective MIDI note in the Keygroup Mapper. Most notes in a drumkit contain multiple samples ("keygroup"). To move a keygroup from one note to another, select all the samples in the keygroup (drag a square or shift-click to select multiple samples), then double-click in the 'High Key' field in the upper left corner of the Keygroup Mapper. Enter the note that you want to move the keygroup to, then double-click in the 'Low Key' field and enter the same note number (unless you want to map the keygroup across a wider range of MIDI notes).

Please refer to p.58 in the MachFive manual for further details.

Alternately, you can remap the keygroups from the List Editor. To display the sample data as a list, click on the icon at the bottom of the right hand column in the Display Area. The low and high key assignments can be changed here as well. The List Editor is further explained on p.68 of the MachFive manual.