You can delete photos & videos that are no longer needed from the Photos and Albums sections as well as files & documents you no longer need from the Files area. To do this, follow these steps:
Select the corresponding files by clicking the checkmark in the upper right-hand corner of each thumbnail. Then a blue navigation bar will appear at the top of the screen showing several commands. Click on “Delete” and confirm your selection. Your deleted files will then be moved to the Trash.
Alt: Selecting files for deletion
You can find the Trash for photos & videos in the Albums section; the Trash for files and documents is located in the Files section.
Here you can read more about how you can restore data that was accidently deleted.
You can learn how to permanently delete your data here.