For editing build.prop you'll need a file explorer like Solid Explorer, Root Explorer and so on.
is located in the root /system.
0. Make a backup of your original build.prop before start any modification.
1. With your file manager, go to /system and find the build.prop file and copy this to yourSD card.
2. Make a copy of this original file and sore it somewhere in case you mess up.
3. Either connect the phone to the PC or use a card reader to access the build.prop file on the SD.
4. With the text editor, open the file and add one or how many you want of the following text at the end :
Battery Tweaks for power save:
Tested by me on my S4 mini and my power consumption over night in about 10 hours is only 1% with wifi and 3G off.
Explanation: If you don’t keep Wi-Fi active lowering the value below the default of 90 seems to make Wi-Fi connect faster after being turned on but will consume more power if turning Wi-Fi off has been forgotten when not in range of a known network. (my personal set up for wifi supplicant is 240)
Explanation: Minimize the time of waits until it rings on a call (not really necessary if you're not experiencing problems). Not tested yet.
Explanation: Disable bootanimation for faster startup. Tested by me.
NOTE: If you want some more tweaks, but test for yourself, then visit the thread bellow or search over the WWW:
[TWEAKS][SCRIPTS] Collection of 'em all - build.prop; init.d; etc.
Relevant article from XDA Portal:
Build.prop Tweaks: Placebo or Actual Development? See for Yourself
or original research thread made it by @LaraCraft304
from here: [Guide] demystifying build.prop [Valid entries]
PS: If I test some other tweaks I will update the thread soon.