Update Android on HTC Evo 4G

I have a rooted FreedomPop HTC Evo 4G PC36100 with CyanogenMod 2.6.37.6. My Android version is 2.3.7. Beseband version is 2.15.00.01.15.

How can I update Android to the latest version? Is it possible? There's no button in the Settings to update, so I assume I will have to use some manual method.

I use Linux Mint 17.1 Rebecca. I installed Fastboot and Android Debug Bridge.

Maybe there's a custom rom I could install that would update the Android version? Any tips would be appreciated.