- Boot into Manjaro on usb stick
- ctrl+alt+t
sudo manjaro-chroot -a
grub-install <partition>
(eg: /dev/nvme0n1p6 or /dev/sda)grub-install --recheck <partition>
update-grub
exit
reboot
(remove usb stick, grub should re-appear with Manjaro as a boot option)
Grub may not add Windows (or another boot option) to the menu. Boot into Manjaro.