Welcome to the Forum @mag 
I don’t work for SparkFun, but I’m glad to share a few of my notes from upgrading the PostCard’s Firmware.
Download and install QGNSS, per the GitHub issue you linked to.
While this is mentioned in the Guide, you will also need to have a Serial Terminal connected to the port that includes “Serial A” in the description to Reset the LG290P within 20 seconds of uploading FW in QGNSS.
Here’s my notes:
Updating LG290P -
Use QGNSS V2, select the com port that mentions Serial B in the description, Baud rate is 460800
Also have a serial terminal connected to the port that mentions Serial A in it’s description, Baud rate is 115200 - this is how you reset the LG290P within 20 seconds of Flashing with QGNSS. Go ahead and navigate to the Debug Hardware menu in your serial terminal of choice and be ready to send command 13 after you flash with QGNSS.
Clip from the Git you linked to:
To update the PostCard’s RTK Everywhere firmware is very straighforward.
Use the RTK Uploader Tool, select the port that mentions Serial A in it’s description, Baud rate is 115200. Download the latest .bin from here, which is currently RTK_Everywhere_Firmware_RC-Jan_20_2025.bin as of today, and flash it.
Note, the PostCard will work right out of the box (and without the Shield). It just wont send GST messages until you update the FW. I decided not to use my Portability Shield, so I didn’t solder the headers. I’ll let someone else help you on that one 
Summary:
To monkey with LG290P => Serial B, baud rate 460800 (this will be rare)
To monkey with the PostCard => Serial A, baud rate 115200 (update PostCard FW)
The PostCard is a brand new Product. I’m sure the documentation will catch up soon to add the LG290P FW flashing procedure, and the Shield install.