Translations:GRUB/Restore the GRUB Bootloader/10/en
Use manjaro-chroot
Manjaro deploys a script called manjaro-chroot takes an optional argument which will search the visible devices - scan the partitions for signs of an operating system. If more than one Linux operating system is found you will get a choice of which system to chroot otherwise the file /etc/fstab from the system is used to mount the partitions and chroot into this system.This script is only available in live iso by default but you can get it in an installed system by installing manjaro-tools-base package.
root # pamac install manjaro-tools-base
root # manjaro-chroot -a