Hello and welcome to just answer
Sorry for the delay in answering your question.
To remove the aerial from your Q3 you would need to lower the rear section of the head lining to allow you to gain access to the wiring and securing bolt. There is a bolt in the middle of the aerial base that you need to unscrew and is No3 in the picture below.
The picture also shows the two wires that you need to disconnect to allow you to remove the aerial.
The issues that you may run in to with finding a replacement is to make sure that the connectors are the same and if you have Sat Nav in your car this is also usually part of this aerial.
If you require further or some more information please feel free to ask