Yeah, you can change it (custom roms), but the one pre-installed is best.
If you are looking to buy a phone, don't go for Qmobile.
Restart the phone. It should help, as it just looks like a temporary software bug.
If it doesn't. Post the full battery stats, battery graph and also screen on time.
Also run the device in safe mode for few hours. If battery timings improve, battery drain is due to some corrupt user installed app. If battery timings stay same (no improvement) even in safe mode, then it could most probably be phone software, or even hardware issue (like damaged/dented battery).
Don't install clean master or any battery saver, it ruins the battery timings even further.