How to mount /system rewritable or read-only? (RW/RO) How can I mount the /system directory rewritable or read-only on my Android phone?
There are a few methods how you can mount your /system directory RW or RO. However, it will require root.
Method 1:

Connect your phone to the computer. (Make sure USB debugging is enabled on your phone)

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.



Type this:

adb shell

su

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:

Open terminal on your android phone (download here):

Type this in the terminal:

su

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






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