You can use
to find the UUID (it will list all partitions) and
mount -o remount,rw /
to mount the root partition in read/write mode.
Here an example for a correct fstab:
# /etc/fstab: static file system information.
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
PARTLABEL=UbuntuStudio1804 / ext4 errors=remount-ro 0 1
PARTLABEL=Swap none swap sw 0 0
PARTLABEL=ESP1 /boot/efi vfat umask=0077 0 1
PARTLABEL=DATA /media/DATA ext4 rw,noexec 0 2
PARTLABEL=data /media/data ext4 rw,noexec 0 2
PARTLABEL=StudioBackUp /media/backup ext4 rw,noexec 0 2
/dev/disk/by-label/archiv /media/archiv ext4 rw,noexec,nofail 0 0
I use PARTLABEL instead of UUID because this is better readable, the important thing I want to show you here is the order of fstab-lines, that's relevant, fstab-lines are processed in line-order, so you must mount
/ first! Also note that lines starting with a
# are comments and not processed.