Read/mount external sdcard to /mnt/sdcard/ or /storage/sdcard0

how to make phone read and mount external sdcard to /mnt/sdcard/ or /storage/sdcard0/??
because by default it mounts it in /mnt/sdcard2/ or /storage/sdcard1/
the phone storage is the one mounted in /mnt/sdcard/

please help. how to do this without using any app. just by editing a system file. please help :D