Gnome Standard is the default UI. To change it, click the gear-like icon during your first login and choose "classic" instead of "standard":
For more information about Gnome Classic and Gnome Standard please check the following articles
- Gnome Classic: https://help.gnome.org/users/gnome-help/stable/gnome-classic.html.en
- Gnome Standard (Gnome Shell): https://wiki.gnome.org/Projects/GnomeShell/Tour