In Arduino choose FILE > OPEN, then navigate to the firmware you downloaded : Firmware -master > Firmware -master > Repetier Firmware > Repetier > Repetier (this file has the Arduino Icon next to it) Select the file and choose open.

With the firmware open in Arduino, click on the configuration.h tab near the top center of the screen.

Verify that line 42 matches this: #define MOTHERBOARD 302

Change line 50 to match this: #define PRINTER 4

Save the changes & Click the compile button (check mark in the upper left corner of Arduino)

Click the upload button (right facing arrow in the upper left corner of Arduino)

The firmware has now been uploaded to the Mini RAMBo board.