OK , looked on the net and Microsoft has dropped Silverlight from Microsoft Edge , if you want to watch BT SPORT on windows 10 you have to download another browser.
I hope BT are working on sorting this out , after all i do pay for BT SPORT and i don't want to download another browser.
Well - assuming you upgraded to Windows 10 - your existing applications including your existing internet browsers should still be available - so you don't need to download them again.
I also beleive that Windows 10 is shipped with 2 browsers Edge and Internet Explorer - if you are using Edge then clicking on the the 3 dots ... in top right hand corner brings up an option to open the current web page in IE.
It is straight forward to change the default browser if you wish.
If you hunt around, Internet Explorer should still be hiding somewhere on Windows 10. Edge is now the default browser, but not the only one.
Alternatively, Firefox still supports Silverlight.