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It would be best to let Android handle it. This OS is different from Windows, meaning that the reverse is true: Empty RAM is wasted. Now, something you can do is disable stock apps you don't use in the Application Manager to free up more RAM. But this is not to have RAM sitting free, it is for your use when a large app, like a game, needs it.
Ignore it. The Android system will handle it properly. If you are worried though just do what I do... Every night before bed I reboot the phone, plug it in, sleep. Then good to go the next day .