You need to edit your current style or identify a style which has your settings. I should warn you that most styles would never advocate chaning Author1 and Author2 to author et al, but it is easily achieved. Most would only apply it to 3 or more and not 2 or more. But to change it to whatever you wish, you simply need to edit your numbered style (Edit>Ouput Styles> edit “the currently selected style name” (see image 1) and then change the Bibliography Author list settings (highlighted in image 2) and change the current number 4 to 2 or to 3.
Then save as, and give the new style a new name and then in your document, change the style to the newly created name (you may have to view additional styles to find it the first time).