iCloud, want to go back.

I have a iPhone 6s, the memory is 16GB, and then I usually just have 5GB on iCloud. My current operating system I iOS 11.0.3 And then a couple of months ago I bought some more icloud space, up to 50GB. But it turned out that I did not need the space after all. I have tried, and I went in on "iCloud" and then it says something like "change space plan" or something, but when I go in on they it says that I only have 5GB but I pay for it every month.

So know all I want to know is how to go back to 5GB and stop getting a monthly charge. I just cant find how to do it.

Thanks.