Hello there,
To remove the top you need to power the unit and open the lid, leave it in the up or open position. Then power down.
To the left and right corners of the inside of the lid are 2 little plastic tabs. One at a time gently lift each little tab and carefully start to slide the lid up and off.
Then check all screws are removed from the rear and you should be able to lift the rear of the plastic cover and carefully slide it away from you and off.
Take care with all the internal plastics. I’ve changed belts on Beocord 3500 models and found plenty of brittle and breakable plastic pieces inside.
Julian
(Martin from this site has a good paper guide for belt changes on Beocords)