NANO : Problem about modify extlinux.conf

I have a problem about nano:  I am working in emmc, but I want to work in SSD for more space. I copied my filesystem to SSD, modified the extlinux.conf in /boot/extlinux/  to set the root=/dev/nvme0n1p1 and then reboot, after I logged in, I found it still work in emmc, /proc/cmdline still show root=/dev/mmcblk0p1.
I modify APPEND many times , it doesn't work, but I modify LINUX ,it works.
By the way, in NX,  this method I used to put my filesystem in SSD is ok, so I don't know what is different and how to rewrite extlinux.conf correctly.

Please follow this method.