You will need to flash Clockwork Recovery or TWRP to your phone using Fastboot. After that you can then install a custom rom. XDA Developers have a huge list.
Here's one for the phone that's Android 5.1.1
http://forum.xda-developers.com/show....php?t=2799765
When flashing a new ROM, you will lose EVERYTHING and have a good chance of bricking your phone. From one of the recoveries it's recommended that you do a full backup.