If you receive a notice that alleges file infringing and you do not accept that you have done so, you can challenge the notice.
The way to do this is explained in the notice, and involves you replying to the organisation that sent you the notice (your ISP) using whatever system they have. This may be by return email or via a form on their website.
You also have the right to respond directly to the copyright owner or their agent. We recommend you do not take this approach. If you reply through your ISP, you remain anonymous to the rights holder. If you contact them directly, they then know who you are and have your contact details, and could send you notices outside the protections set up by the filesharing law.