https://btcloud.bt.com/ from a browser.
There is a Windows 10 app but the last update caused so many problems that have not yet been resolved that I wouldn't recommend it.
I tried the cloud link but it just tells me I need to log in with the primary BT ID associated with my broadband account. When I click the log in link and try to log in it just sends me back to the error page and tells me I need to log in.
Yes you do need to login but having done so it should take you to the cloud.
If you still can't log in there is a problem with your account
I finally manged to access BT Cloud. For anyone else wanting to use it here's how I did it. I googled BT Cloud and went through the results until I found someting that told me how to activate it.
Log in to BT
Go to My BT products
Scroll right down the page until you see a panel with BT Cloud.
Click on actrivate.
From there it wasn't clear what to do next so I googled BT Cloud login and it clicked on a link that took me to the BT Cloud page and from there I could access the cloud storage.