First u need to root your phone. It is fairly easy,takes about 5 mins but if u dont know much about rooting watch some videos on youtube (thats how i learned to root my phone). After rooting, download rom manager from playstore. You have to install both premium and free version of it. Then make backup of you current rom. You will see back up current rom in rom manager, if you do something wrong while installing new rom, you can use backup. Then,in rom manager, there is download rom. Download cyanogenmod night and install it. Be careful while doing this coz you might brick your phone. So watch some videos. Let me know if you need any more help.