Samsung Galaxy S Plus (GT-I9001) was one of the best devices running on the Android 2.3 (Gingerbread). It spots a 1.4 GHz Qualcomm Scorpion processor with 512 MB of RAM, so the latest Android 5 Lollipop should work smoothly on this device. The CyanogenMod 12 ROM is the best choice for you to get Android 5.0.1 experience on your Galaxy S Plus.
Samsung Galaxy S Plus (GT-I9001) users can now enjoy Android Lollipop via unofficial CyanogenMod 12 ROM. This custom ROM brings the new Material Design interface and other Lollipop features to Galaxy S Plus. However, the ROM is still in its early build, so users should not look forward to enjoy the complete OS just yet.
Before you start installing this custom ROM on your phone, please note that, there are a number of bugs on this ROM such as slow call UI, Video recording at 720p quality, some apps force close, etc. however future update may fix them all.
WARNING: This procedure will void your device’s warranty. Follow the instruction at your own risk. We will not be liable if any damage occurs to your device and/or its components. It is recommended for technically talented users only.
Important Note: As always, backup any important file stored on your device before flashing the ROM.
How to install CM12 on Galaxy S Plus
1. Copy the downloaded TWRP recovery, CM12 ROM and Gapps to external SD card of your phone.
2. Boot your device into recovery mode and flash the TWRP 22.214.171.124 recovery. Then reboot your phone.
3. Reboot your phone into TWRP recovery again.
4. Wipe Data as ext4 partition (Wipe -> Advanced Wipe -> Select Data -> Repair/Change File System -> Change File System -> EXT4 )
5. Wipe Dalvik Cache, Cache, System and Android Secure (Wipe -> Advanced Wipe)
6. Now, flash the ROM package and then flash Gapps package.
7. Reboot your phone and enjoy Android 5 Lollipop.
For your information, CyanogenMod 12 is the most popular, AOSP based custom ROM built around stock Android 5.0 Lollipop. However, CyanogenMod 12 (CM12) is still in works, so you will find many CM features missing in this unofficial build. (Source – XDA)