It is hard to believe but it is true. XDA developers has released cyanogen mod 12.1 ROM for the HTC Desire S which is basically Android Jelly bean 5 for. Installation instructions are as follows. Just make sure you have custom recovery for HTC Desire S onboard and have unlocked it through HTC Dev page.
Download the latest version of ROM from here and gapps from here. Put the files on SD card; Reboot to recovery; Do factory reset (ie. format /system, /data, /cache, /sd-ext and /sdcard/.android_secure); Flash cm-12.1-YYYYMMDD-UNOFFICIAL-saga.zip; Flash gapps zip; Remember to flash boot.img via fastboot if you have S-ON; Reboot and enjoy android 5 on your HTC Desire S.
In case your phone does not boot, then you can install using ADB. You can get it by installing android SDK platform tools. Reboot into booatloader, open CMD and navigate to the location you extracted your boot.img and type:
fastboot flash boot boot.img