To delete a historical version, first click on the file in the View tab. A window will slide out from the right displaying all of the file's historical versions, including the date the version was saved and its size. Highlight any of the versions and then click the 'Delete' (red 'X') button to remove the versions from your account. To select multiple files, hold down the Control key and click which files you would like to remove.
You can also purge historical versions from the command line. For instructions on how to do so see please see How do I use the --purge-historical-versions option?