Select the 'Restore' tab, select either 'Image Restore' or 'File and Folder Restore', select the file you want to delete, click 'Other Actions' and select 'Delete'.
Please note that the 'Delete file' option will only be presented for a backup set.
This opens the 'Delete files from backup set' dialog to safely delete backup files:
Along with the select file, any dependant files will automatically be included for deletion. In the above example Incremental image '9427874FB039F667-57-57.mmrimg' was also selected because it is dependant on the previous incremental 9427874FB039F667-56-56.mrimg.