You need the Audi Certificate of Conformity, the DVLA will then update your V5 to reflect this.
However, the V5 isn’t the legal document in this respect, it’s the VIN plate on the car if you were pulled at the side of the road although as the A2’s is hidden a V5 updated would be helpful. The VIN plate should have four figures on it, they are;
Gross Vehicle Weight
Gross Train Weight
Front Axle Limit
Rear Axle Limit
As such, it doesn’t show a ‘towing limit’ but the basic principle is the first weight taken from the second is the towing limit. Legally, so long as the second limit isn’t breached then you can tow higher than this (ie so long as the car is not full). If the first two figures are the same or the second figure is empty this does need rectifying.
It gets more complex with unbraked trailers as it’s 750kg or 50% of the Mass In Service (whichever is lower) as a maximum but some manufacturers have lower figures.
If your VIN plate is correct (might be worth posting a picture with the VIN number hidden if you aren’t sure) I wouldn’t bother changing the V5; my family car has a towing limit of 2000kg, the VIN plate is correct but the manufacturer had a series of cars in 2018 that were incorrectly registered with the DVLA; mine has a braked limit of 750kg showing on the V5. I will probably get around to getting it changed but legally its the VIN that matters for dragging our 1500kg shed around.
Hope that helps.