Try checking the treo wiki and read through everything before trying to perform the rom upgrade. Remember that a simple mistake can and might render your device useless.
"Insert the reader w/card into your USB, after the PC recognizes the card right click on the drive letter. Select Format from the options and make sure FAT32 is selected as the file system then select start.
Also, you need to copy the files to the memory card one at a time; DIAG file first and then delete the DIAG prior to loading the IMG file second.
Step 1:
-Copy the ‘CheeDIAG.nbh’ file to the root directory of a miniSD card. Note: Windows Mobile recognizes updates only from FAT32 formatted cards.
-Insert the card into the device.
-Press and hold both the PTTApp Button (lowest button on left side of device) + Pin Reset (next to miniSD card slot). You will be prompted to press the ‘End’ (red power) key to start the update. This step takes only a few seconds to complete. Once the flash is done, you will see: ‘Model ID: Palm Treo 750’ displayed by the device.
Step 2:
-Copy the ‘CHEEIMG.nbh’ file to the root directory of a miniSD card. Note: Windows Mobile recognizes updates only from FAT32 formatted cards.
-Insert the card into the device.
-Press and hold both the PTTApp Button (lowest button on left side of device) + Pin Reset (next to miniSD card slot). You will be prompted to press the ‘End’ (red power) key to start the update. This step takes about 5 minutes to complete.
-Hard reset the device once the device indicates the upgrade has been completed. To hard-reset, hold the ‘End’ (red power) key while pressing the pin reset button. Answer “yes” to “Erase all data” prompt.
Step 3:
Verify the device software via the device <start><settings><system tab><about><phone tab> The device should display firmware version on the resulting screen."
Good luck!