Well it would depend on the files and how you are trying to delete them. If they are say photos, you should be able to delete them through the gallery. If other files (or also with photos) you can use the file manager app on the phone. Or if there isn't one on the device you can download one from google and locate and delete the files.
There is also the option of connecting the phone to a computer via USB cord and accessing the files from a computer to choose what to delete.
Should none of these get what you want, and if you are OK with removing all data off the card, you can always format the SD card.