Hello archie.
The best idea would be to contact your local Bang & Olufsen Retailer and let him change the fabric cover.
If you really want to do it yourself you should look at the fabric cover from the back. There are little "hinches" where the fabric is being hold in place and under tension. Pull or cut it out of there and it should come loose.
But to be honest, I can not give you a better answer.
Regards
Alex