You might have been fed up with your current Android version and want to enjoy some new features that the others have, or update to a new Android version which your OEM didn't offer yet, then it's time to flash a new Android version (or ROM) for better experience.
Actually the ROM is just the same as the Windows operating system which have different versions, no matter it is stock ROMs from Google or manufactures, or custom ROMs from third parties. So flashing a new ROM equals to install a new operating system to your phone.
According to the poll on the Google Plus, just 43% users rooted their devices, we can benefit a lot by rooting our devices and flashing a custom ROM is the most important reason for a lot of users, but also we will face a lot of rooting risks such as void the warranty, while if you finally made the decision and were ready to flash a new ROM to your Android device, please follow the step by step tutorials as below.
Because the unlocking process is different for every phone, we can not go through the details step by step, please search how to unlock your specific phone in Google. Then download the recovery (TWRP or ClockworkMod) to flash it.
Note: Unlocking bootloader and flash a custom recovery didn't mean rooting your device, it's actually the first step before rooting. But we strongly recommend you to root the device to backup the data.
The first thing you need to do after flashed the recovery is to backup everything you got in your device. We should make a Nandroid backup which is the complete image of our current system. If something goes wrong, we can restore our device to the first place where we started the rooting. We can save a lot of troubles if anything got wrong which actually happens very often. Here are the simple steps to make a Nandroid backup.
If you don't want to install all of the apps one by one and lose app settings after flashing the new ROM, you can backup all the apps and also settings firstly by using the app Titantium Backup which requires rooting. That's the reason why we strongly recommend you to root your device firstly. This Android backup tool could also help you backup all your Android files to PC with one click.
Firstly you need to download the ROM you need which is a zip file, here are the 6 most popular custom ROMs you can consider. After downloaded the ROM, copy it to your SD card and start to flash it.
And remember to restore all the apps and settings to your device if you made Titanium backup. After that, it's time to enjoy the new shinning ROM.