If they don't, they won't be able to receive a number of TV channels including Five and ITV4, and will also lose BBC Radio stations. Retuning will restore these services.
It's all part of the preparations for the digital TV switchover, already started in some areas and due to be completed in 2012, and will ensure Five is as widely available as the other main channels after switchover.
Technical preparations for future improvements to the Freeview service, such as Freeview HD, will also be made on Wednesday 30 September.
For more information and retuning advice, see the Remember to Retune website.
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