Turn off supply to cistern and disconnect tail
Disconnect overflow (if fitted)
Undo cistern bracket (usually wingnuts at rear of pan)
Undo screws holding cistern to wall (if fitted)
Lift off cistern
Replace doughnut washer
Refit in reverse
If you're not sure how to tackle this, watch the clip on the link below. Although this is predominantly about changing the flush valve (you can ignore that bit as you are keeping yours) pay attention to the bits where you can see him remove and then replace the large doughnut washer.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local