You are trying to upload a new favicon file on the Site Settings > Site Info area, but are encountering some issues such as being unable to select your file for upload or getting an error after the upload due to the format.
A common reason for the favicon file not to show as an upload option or failing to upload is when one uses an invalid format such as an SVG image. Note that only ICO format files are accepted for the favicon area. Thus, the solution is to convert your existing file into an ICO format and upload it again.
In case you are already using an ICO file and still encounter problems with the upload, it is possible the current version used is 9.6 which has some issues with uploading the favicon. For those scenarios, a workaround is to upload the ICO file to a folder in the Portal from the Content > Assets menu, and then select the file as the favicon using the 'Browse File System' option.
Alternatively, one can also follow the first method from the Updating the Favicon in a DNN Instance article in order to update the default favicon through the file system.
Once the steps above are completed, the new favicon will be displayed in the browser.