1. Go to a webpage 2. Open Web Inspector 3. Go to Network tab 4. Ctrl+Click one of the resources on the left tab 5. Choose Create Request Local Override You can modify the body of the Request if there is one. (HTTP POST or PUT for example) but you can't modify the HTTP headers. There are cases someone wants to modify the HTTP headers to test a variation of response depending on the new header. For example, testing that a site reacts differently if only the HTTP User Agent is changed for one single resource.