There are two ways to select a group of files inside a folder:
1. With the Keyboard:
Use the arrow keys to navigate items, and CTRL and SHIFT keys to highlight a group of items. Then press "ENTER" and the highlighted items will change their state from non-selected to selected, and vice versa.
2. With the Mouse:
Simply drag the mouse over the items you want to select. The items will become highlighted and change their state to selected, or non-selected if already selected.