A common problem that every Android user encounters, is the gradual drop in the gadget's functioning speed. One of the best softwares to fix parts of this problem for you is Clean Master. Download this application form Google Play and install it on your device.
When you open the app on your phone, the first thing you see is a message like in the screenshot below. If your device is rooted, press Ok. Otherwise, you can choose the "Memory Boost" option.
Then the page shown below will appear. Choose the "Boost" option so you can free a big part of your RAM.