There are a few methods how you can mount your /system directory RW or RO. However, it will require root.

Method 1:

  1. Connect your phone to the computer. (Make sure USB debugging is enabled on your phone)
  2. Open CMD/Terminal on your PC.
    • Windows: CTRL + R, then type cmd.
    • Ubuntu: CTRL + ALT + T.
    • Mac: Navigate to /Applications/Utilities/ and double-click on Terminal.
  3. Type this:
    1. adb shell
    2. su
    3. Choose one: (for security mount /system back to RO when finished)
      • Mount system RW: mount -o rw,remount /system
      • Mount system RO: mount -o ro,remount /system

Method 2:

  1. Open terminal on your android phone (download here):
  2. Type this in the terminal:
    1. su
    2. Choose one: (for security mount /system back to RO when finished)
      • Mount system RW: mount -o rw,remount /system
      • Mount system RO: mount -o ro,remount /system

Method 3:

If you don’t want to type the command every time in the terminal, I’ve written an app designed to do this for you:

Android 2.3

For people running Android 2.3 and the command failes, look at this answer: https://android.stackexchange.com/a/125437/95577