Here's my old "Upgrading your PIC made easy" guide:
Required:
Minimig
Null Modem serial cable – a normal serial cable will not work unless it has a null modem adapter.
PC with a serial port (USB serial adapters work OK)
Tiny Bootloader – free software downloadable from the Internet
#1: Connect the Minimig to an appropriate serial port on a PC using your NULL MODEM serial cable.
#2: Set jumper on Minimig to from “FPGA” to the “MCU” position.
#3: Switch the Minimig on.
#4: Download and run the Tiny Bootloader program on the PC.
#5: Select the correct COM port and rate (115200)
#6: Press the "CheckPIC" button on the Tiny Boodloader display.
#7. Quickly press “PGM RESET” on the Minimig motherboard within FOUR SECONDS to establish communications between the Minimig and Tiny Bootloader on the PC.
If you succeed you should see something like the following message in a log window:
Connected to \\.\COM1 at 115200
Searching for PIC ...
Found:18F 252o/452o
If you don't you will see:
Connected to \\.\COM1 at 115200
Searching for PIC ...Not found,
ERROR!
If you get the error repeat steps #6 and #7.
#8: On Tiny Bootloader select the hex update file that needs to be written to the PIC.
#9: Select "Write Flash" on Tiny Bootloader and immediately press the reset button on the Minimig motherboard.
If you succeed you will see something like the following message:
Connected to \\.\COM1 at 115200
HEX: 6 days old, INHX8M,18Fcode, total=14552 bytes.
Searching for PIC ...
Found:18F 252o/452o
WRITE OK at 20:55, time:7.173 sec
MAKE SURE YOU LET TINY BOOTLOADER FINISH WRITING TO THE PIC BEFORE YOU UNPLUG THE MINIMIG!!!
#10: With the Minimig switched off, move the “MCU” jumper on the Minimig back to the old position “FPGA”
