mount partition some where. Bind dev, sys and proc. Then chroot to the partition and run update-grub inside. Maybe mount windows partition inside the chroot env to make grub find the windows

sudo mount /dev/sda2 /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /sys /mnt/sys
sudo mount --bind /proc /mnt/proc
sudo chroot /mnt
sudo mount /dev/sda3 /mnt
sudo update-grub