The service manual won't tell you how to replace the belts.
I suggest you ask the belt supplier for mounting instructions.

And if you bought off Ebay or the likes, there's more than a fair chance that the belts you received won't fit, so if you are looking at them and cannot
really see how it's supposed to go together, the problem is not necessarily you.

Martin

Thank You Martin,

yes I got them from eBay but I bought them from a trusted seller, so think they will fit.
I was just looking for instructions how to open the Beocord, what screws need to be removed and so.

But I simply will give it a try next weekend. You can see the belts on the backside of the Beocord threw the grill, so I will try to remove the backside and have a closer look.

Georg

So I wanted to report, that I was successful.

The main belt was still in takt, the small front one was loose and the third one on the back was torn.

The two thin ones were quiet easy to change, for the main belt I needed to remove the tape unit.

Now everything is up and running again.

I made some photos:

Open Beocord, I also took of the back panel but it isn't really necessary, since the tape unit has to be removed anyway.
To open the front, only losen the 4 screws on the top side, don't remove them. 

Toren belt on the backside.

The small belt on the front was also lose.

Replaced one.

To remove the tape unit, you need to losen the upper mainboard and disconnect 3 cables in total.
It's a little bit tricky to get it in and out, but in the right angle it's no problem.

To get to the main belt, you need to remove the cover on the bottom side of the tape unit. It just covers it, nothing else comes lose.

Supplied with new belts. Since the thin one was toren, I had no idea how to wield this belt, but I figured it out.

Put everything back together and your Beocord is good to go 😁