For those who want to change density without root, follow these steps
1)Open settings go to about
2) Tap build number 6 times til lit says youre now a developer
3) Go developer options, enable usb debugging
4) Download adb
here
5) Connect your phone and inside adb command prompt type adb devices
6) command prompt will say unauthorised. Press ok on your phone
7) Type adb devices again, now it will show as authorised
8) For ze550ml you can type:
adb shell wm density 240
For ze551ml you can type:
adb shell wm density 380
9) Restart phone and enjoy bigger screen area
To undo changes type:
adb shell wm density reset