#Downgrade kernel and update boot ARCHLINUX

#-Restore the GRUB Bootloader
Chroot into your existing arch Installation
lsblk -f
or
sudo blkid -o list -c /dev/nul
mount /dev/[partition used for arch system] /mnt

IF you have used a separate partition for your GRUB bootloader, then this must also be mounted. The syntax to mount a separate partition used to boot is:

mount /dev/[partition used for GRUB] /mnt/boot

cd /mnt
mount -t proc proc /mnt/proc
mount -t sysfs sys /mnt/sys
mount -o bind /dev /mnt/dev
mount -t devpts pts /mnt/dev/pts/
chroot /mnt
 1-restore grub
sudo pacman -S mtools os-prober
grub-install --recheck /dev/sda
update-grub
 2-(if you want to downgrade the kernel)
uname -r   (to check kernel version)
pacman -U /var/cache/pacman/pkg/linux-XX.tar.xz
#
mkinitcpio -p linux
grub-mkconfig -o /boot/grub/grub.cfg
umount -R /mnt
reboot