I lost all details of my old apple ID but I am still paying subscription for icloud. How can I cancel?

So, I think you're best bet may be to address/figure out out your Apple ID issue first. Try this if you forgot your Apple ID. Try this if you forgot your Apple ID password. You can reset your Apple ID at iforgot.apple.com.

Then, once you know that you can do whatever you need, like see and your subscriptions and change whatever subscriptions you want to change or see or cancel the subscriptions you have.

Lastly, you could try Apple Support options, but I understand they are fairly swamped right about now, so I'd try the foregoing self help first if possible.

Good luck!

JDee