Matt Redstone — Edited
Click the first slide, scroll down to the last one, press the 'shift' button, click the last slide, and everything should be selected. Then hit delete- Also, Leanna Van Zee, if there are any you want to keep that are in between the first and last slide as Matt Redstone mentions, you can hold down CTRL (CMD on a Mac) and click each that you want to keep and then hit delete. This will UNselect those that you want to keep.