Most commands are available from the menus, the toolbar and from the popup-menu. The popup menu is specific to the view and can be popped up with a right-click on the mouse. On the Macintosh ctrl-click will show the menu.
Most of the commands that change the view can be undone. Once undone, the command can again be redone. A whole sequence of commands can be undone one after another. The sequence is broken if a command is executed which cannot be undone. Then, unfortunately, there is no further way back. The tooltip on the undo/redo button will show the next command it will undo/redo and the status bar will also show information about the command.
Undo / Redo functionality is available from the Edit menu and from the toolbar:
Undo
Redo