First, navigate to the Admin 2020 settings page. This is in the menu under settings or you can click on settings underneath admin 2020 in the plugins table.
The first page of this settings page is where you can activate or deactivate admin 2020 modules. Go ahead and disable the theme, admin bar and menu modules and press save.
Once you have saved your modules, navigate to another page or refresh and you will have the default UI back! You can still use admin 2020 folders, content page and overview page as well as all the other features but all while keeping the default WordPress UI!