Your canceling was probably too late to show it in their bill. You will have the costs of the subscription returned within some days. There is no action required.

I cancelled it in August and they did return the cost of the subscription in August. 

Then for the whole month of August I had zero subscription fees. 

But starting September it shows that I have a few of 5.6 even though it has been changed since the previous month

Do you mean that it shows 5.60 euros subscription costs in the widget in Mijn NS and the app that also shows the travel costs and when your next invoice is? This widget is known to be buggy, especially after a change of subscription.

If this amount is not actually being taken from your bank account, and it doesn't show up on an actual invoice that can be found in the My payments section of Mijn NS (the website, not the app), ignore it. If it is taken from your bank account, then you should contact customer service.

I have literally zero trips in August but NS charged me 252,00€. How can I get this money back?

In Mijn NS (the website, not the app) you can find a detailed specification for every invoice in Betaaloverzicht/My payments. That will show you why you were charged this amount.

If after checking the invoice you still feel you have been charged too much, or if you can't find the detailed invoice, then you should contact NS customer service directly. They can also tell you why you were charged this amount and correct an incorrect invoice. This can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on X (formerly Twitter) or Facebook.