In ribbon mode select the last ribbon tab View and click in fourth group Frames on first command Flip frames. The default hotkey assigned to this command is Ctrl+Shift+F4 and can be changed in Key Mapping configuration window.

In toolbar/menu mode click in menu Window on the third menu item Flip First <--> Second.

Christophe wrote:What is the toolbar/menu mode?

Click with secondary (usually right) pointing device (mouse, touch pad, ...) button anywhere on the ribbon for opening the context menu and click with primary (usually left) pointing device button on last context menu item Toolbar/Menu Mode to switch to this user interface mode with a classic menu at top and toolbars below.

Click with secondary pointing device button on the menu and click with primary pointing device button on last context menu item Ribbon Mode or click with secondary pointing device button on a toolbar and click with primary pointing device button on first context menu item Ribbon Mode to switch back to the ribbon user interface mode.