I decided to change the Android OS on my phone. This is somewhat popular these days because Android is an open source OS. Most phones have a modified version of Android full of software a phone carrier or manufacturer puts on. Think of the crapware you get on a PC you buy, that's how obnoxious it can be. These carriers and manufacturers also are known to not update the OS on an older phone, leave phones Sometimes open for security risks. So the option is to install a new version of the Android OS, usually the open source version.
In the past, this wasn't the easiest thing to do. First you had to gain root access to the file system on the phone, usually done with software you put on your computer. Sometimes this could ruin your phone if done wrong. Once you get this done you put the phone in a state where software can be installed, where you then install the new OS.
Cyanogenmod is a popular OS. It's based on the open source version of Android and is the "pure" version of Android for the most part, and runs faster and more efficient. They recently published an android app that along with a Windows app, installs Cyanogenmod OS on your phone (if it's compatible) with little to no work.
I decided to try it, and it worked flawlessly. After reinstalling my apps and setting up my accounts on the phone (all that is wiped after you install the new OS), I found my phone running much faster. Also without the crapware, I had a ton of more free storage. After using the OS for a week, I am happy with the performance. The version available is not the newest version that just was released by Google, Android 4.4, but 4.3.
Battery life for me is a little better, but not insanely different.
Warnings now about doing this. In the old way, you were able to backup the old OS and data so you can go back to that if something goes wrong or you don't like the new OS. The app way doesn't have a backup option. Also, only certain phones will work with it. Look at Cyanogenmod 's site to see what phones work. As with anything with changing major software on hardware, it can ruin the phone.
If you are thinking of doing this, look up info online, especially sites dedicated to Android and things related to modding phones.