We'll start at the reset button, and work our way around the wiring diagram clockwise.

The reset button reboots the board, this is OK to press if a reboot is needed.

The firmware is erased in one of two ways depending on the hardware PCB revision:

For revisions v1.04 and later it is a jumper which is bridged to erase the firmware.

For all versions prior to v1.04 it is through an erase switch.

The erase jumper/button will erase the firmware that is stored on the main processor. The SD card does not hold the firmware, just some configuration files.

Do not press the erase button unless you know what you are doing. There firmware does not need to be erased for normal upgrading. The process of reinstalling the firmware after it is erased is detailed In fallback procedure #3.