You cannot modify the dashboards directly; changes you make to the dashboards are not saved permanently.
However, if you want to change the dashboards and adapt them to your own use cases, this is perfectly possible in a different way. The only thing you need to do is to save the dashboard that you want to change in a new folder in your dashboards.
To do this, simply click on the cogwheel symbol in the top right-hand corner of the relevant dashboard and then click on “Save as” in the left-hand bar. Now select one of your own folders and you can modify the dashboard as you wish.