trying to delete a system file

[FiveTechSoft]

Once connected with remote adb shell I try this:

px5:/ $ rm /system/mtc/ring.mp3 rm ro /system/mtc/ring.mp3 (y/N):y rm: /system/mtc/ring.mp3: Read-only file system

Is there any solution for this ? Previously I have used this "backdoor" provided by the manufacturer: setprop sys.rkadb.root 1

thank you