Which type of Beogram 1800?

The newer (flat belt);
Take off cartridge and store in a safe place.
Lift off platter.
Push tonearm so that its outer end is over the area where the platter was.
Losen two screws, one each side from below holding the deck alu surface (Beogram out
over the edge of a table one side at a time – don’t turn it upside down),
lift the deck surface plate gently and pull it towards you.
When you can reach below you can take up the small switch for the play button and unplug
a connector for the deck surface control buttons and take the deck surface off.
Now you can replace the belt and you can also inspect the motor bushings (replace them if they are still the original).
Put back in reverse order – watch out that the leads to the buttons aren’t trapped or sandwiched at the right side screw block.

The older is a different story as the old belt is easiest removed by simply cutting it and pulling it out.
The new belt is placed around (under) the platter form above and, from below, using
a hook-like tool, fish it out and lift it on to the idler next to the motor.

Martin