For editing build.prop you'll need a file explorer like Solid Explorer, Root Explorer and so on.
Build.prop 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:
pm.sleep_mode=1 ro.ril.disable.power.collapse=1 wifi.supplicant_scan_interval=180
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)
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.