How to fix the cooling system in my Audi 80?

I recently noticed that my Audi 80's cooling system is not functioning properly. The temperature gauge goes up to the red zone and I have to constantly fill the coolant reservoir. What could be causing this issue and how can I fix it?

It is possible that there is a leak in your cooling system, causing the coolant to constantly decrease. It is also possible that there is an issue with the radiator or the water pump. I would suggest having a professional mechanic inspect and diagnose the problem.