Assuming you're booting amd64 or i386, the procedure would be:
- Backup your root directory. Have alternate boot media capability in the event of finger fumbles.
- Extract /usr/mdec files from the latest snapshot's base53.tgz fileset, and install in your own /usr/mdec, replacing the files there.
- Copy the newer second stage bootloader /usr/mdec/boot to your root directory.
- Install the newer first stage bootloader /usr/mdec/biosboot using installboot(8). If you've never used it before, you point to the second stage bootloader and the first, and your boot drive (sd0, wd1, etc.). Use the man page for guidance.
Example:
# /usr/mdec/installboot -v /boot /usr/mdec/biosboot sd0