This depends on the fault that has been identified by your water company's survey contractor who should have given you advice on the day of their visit.
If you have received a letter from your water company you can contact them using the telephone number provided.
The property survey sketch enclosed with the letter will give you an indication of what is required.
The ConnectRight website also provides guidance on what to do to check for and put right any misconnections - just use the "check your existing connections" guide.