Android’s system partition is by default mounted as read only, but if you are looking to modify some system files or applications, then if you try as is, you will not be able to do so due to restricted user permissions, hence first thing we need to do is mount the system partition as read/write which will allow us to do necessary changes in system partiton.
Command to remount system to read,write
$ adb shell
$ mount -o rw,remount /system
Now, do adb any changes, and remount the system back to read only as,
$ mount -o ro,remount /system
You may also check “How to Install and Uninstall android application as system app / priv-app”