Select Undo every time you're not happy with your last action or you simply want to try something else (e.g. insert a different clipart or change the background). Don't be afraid of making mistakes because you can always undo the last change done to your project.
Redo reverses the last undo action, however, the function appears only if you use Undo before.
The Delete Tool allows you to get rid of selected objects very quickly. Select the tool, choose an object on the canvas that you want to remove, and select the red "X" icon. It'll disappear immediately!
Removing a group of objects: if you want to remove a group of objects, you need to group them first (use the Inspector or triple tap them). Created a group already? Select it so a red border appears, and remove all its elements by selecting the red "X" located in the very middle of the group. Also, you can still select multiple objects to delete by selecting them separately.