The firmware is included and updated with LibreELEC updates and usually there's no need to do that independently from that.
If you really want you can update the firmware files manually, at your own risk:
Download bootcode.bin, start_x.elf and fixup_x.dat from the RPi firmware github repository, rename start_x.elf to start.elf and fixup_x.dat to fixup.dat and copy the 3 files to the /flash partition.
You can also easily do that from Windows/Mac/Linux (can be helpful if you messed it up) as the /flash partition is FAT formatted.
so long,
Hias