I did the same thing. I was able to remove the battery to wipe it and get back in, and the 2nd time I remembered to turn off Bootp. However, in my particular case, something was still wrong, as the PLC would fault and erase its program every few hours afterward. This was still under development on my desk so I fussed with it for a couple of days before I gave up and sent it to my distributor for a new one. Once it ran for about 6 hours and then faulted, but usually it would only run an hour or two.
I only mention this just in case the same thing happens to the OP. My situation very well could have been a defect totally unrelated to my firmware updates, but I never knew for sure.
I recommend using a desktop (or power saver options set to never idle anything) and ethernet (faster = less time for natural disaster) using an isolated network with only your PC, the PLC, known good cables and a good switch, all plugged into a UPS.
If you do it serially it will take a lot longer, so you must make sure your PC stays "alive" during the whole time. I have seen bricks made because a laptop decided to idle a port that it thought was inactive in the middle of an update, so be extra very careful with power options if using a laptop.