How to delete old android OS after update?

I have recently updated my Samsung S4 I9505 to Android 5.0 from its original 4.4. However, after the update my internal storage has massively reduced. After plugging my phone into the laptop, it tells me that i have 705MB of 9GB left. But when I sorted my internal storage files by size, i found out that the largest file is no more than 20MB, I then selected all files, right click and selected properties, it appears that all my shown files added up is no more than 200MB large. So i am assuming that the rest 8GB of space is been used up by my OS. Any ideas on how to delete my old OS to allow more space?
Many thanks,

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