How to update the location of /sdcard link to the external storage? I wonder how to do this.

Now, this is linked to my small internal memory.

I'm an experienced Linux user, but Android shell don't have unlink and rm /sdcard makes me fear.

What I try to do is to symlink /storage/1234-5678 to /sdcard, but I see no way. 

ln -sfv /storage/1234-5678 /sdcard is trying to create /sdcard/1234-5678 unlike linux ln... 

Don't know what I can do
You should run

mount -o rw,remount /
ln -sfvn /storage/1234/5678 /sdcard


Option -n means "Don't dereference symlinks - treat it as a normal file". Even if you're running a linux you should use ln -sfn, though.