To disavow a backlink, you need to use the "Disavow Links" tool in Google Search Console. First, create a list of the bad backlinks you want to disavow. Then, submit the list to Google using the Disavow Links tool. This tells Google to ignore the bad backlinks when determining your website's ranking in search results.

Google's disavow tool allows webmasters to notify Google of poor-quality links pointing to their website that they do not want associated with their site. This helps SEOs tell Google which external links to be considered when Google evaluates ranking factors.

In case you are entirely new to the whole concept of disavowing links, head on over to Google's own advice to read more. As you will see, Google themselves suggest that it is an advanced tool and not one for casual use, advising that it could very well do more harm than good.