You need to bypass the onboard USB-serial converter with a USB serial adapter. Connect it like this.
USB serial adapter UNO
+5 ---------------------+5
Gnd---------------------Gnd
tx----------------------rx
rx----------------------tx
DTR------100nf----------reset
If you don't have DTR on your adapter or this doesn't work, leave the reset pin unconnected and press and release the reset button when you see the red
Overriding Baud Rate : 115200
message.
The setup will look something like this.
It is a bit of a pain to do, so I would use the board where I wasn't going to change the code very often.