mount root partition to /mnt. Bind dev, sys and proc into root part. mount efi partition into /boot/efi of root part. chroot to the partition and run update-grub & grub-install inside.

sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot/efi
sudo mount --bind /dev /mnt/dev
sudo mount --bind /sys /mnt/sys
sudo mount --bind /proc /mnt/proc
sudo chroot /mnt
sudo update-grub
sudo grub-install