Selecting all objects on a sheet in Excel is easy with Ctrl + A.
Objects are things like textboxes, images, charts and slicers.
If you select one object and press Ctrl + A all the objects on the sheet will be selected.
When you select the object make sure the line around it is not dotted. Some objects like text boxes have a dotted line when you are editing the object.
Ctrl key
After you have selected all the objects you can hold down the Ctrl key and use the mouse to click on objects to de-select them.