Note down what settings you've configured.
Format an SDCard FAT32 (not ExFAT or any other format)
Download the firmware version you want.
Extract it and copy the "BlackVue" folder to the blank SDCard.
Insert SDCard into BlackVue and start it up. It should automatically upgrade or downgrade the firmware to the version on the SDCard. Make sure there is constant power to the device as it takes a minute or two and requires a reboot or two of the dashcam.
You'll now have to put in all settings again.
Edit: If going from 2.x to 1.x or 1.x to 2.x I'd suggest re-entering all settings manually as they're rather different. If going 2.x to 2.x or 1.x to 1.x, you can copy SDCARD:\Blackvue\config\config.ini to your computer as a backup, then back on to the SDCard with the updated/new firmware. This way your settings carry over.