Just edit the /boot/cmdline.txt file, and remove the quiet directive.
# cat /boot/cmdline.txt
#dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=PARTUUID=xxxxxxxx-yy rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait quiet splash plymouth.ignore-serial-consoles
cp /boot/cmdline.txt ~/boot_cmdline_bak.txt
sudo nano /boot/cmdline.txt