This site used to link them. For example I searched for my '16 WRX https://www.nhtsa.gov/vehicle/2016/SUBARU/WRX/4%252520DR/AWD#manufacturerCommunications
The problem is after Subaru started locking down their TSB's to only tech net subscribers what seems like Summer 2018 the above link is no longer updated as frequently. Still worth a check.
I assume you hooked it up to the radio directly?
I used a cheap battery charger when doing mine but yes the update supposedly checks for high / low voltage and backs out if there is an issue. Lucky it didn't brick your unit.
Take a look at the PDF page I shared. If a radio model# on that page matches the model# on the face of your radio you could try the update. No promises it will work and won't render the unit unusable so weight the risks.
The instructions say if the voltage rises above 14.5V the update will abort. My original idea was to keep the car on but after reading that I decided against it. You got very lucky that either that is untrue or you had enough load on your alternator that it didn't rise. Had it aborted you may be riding around with a bricked HU. Either way glad it worked.
Glad it worked out, I read everything 2-3 times and really crossed my T's and dotted my I's. I didn't want to be buying a new HU if the update failed.
Yes I was able to get the full service manual. There is a checkbox on the left for service manuals the same way you search for TSB's. Check that, fill out the Model & Year, and a PDF should come up. They are a good size.