If you have a computer simply do it using Finder on mac or your file manager in Windows. But if you wanted to delete or modify a picture you can absolutely. If you can be more specific about what your looking to do I can give you a better and more thorough answer. Basically though if you are on the phone
1. goto settings
2. storage
3. memory card
4. click on what type of file you are looking to edit, I.e apps , pics , movies , music,....
5. go about editing in your normal fashion
6. Either save over the file and only have the updated version or save the update as a new file and have access to both. <- that would apply to pictures and documents mostly but its all pretty straight forward. Hope this helps.
Also make sure you back up your phone prior to doing anything and you can always refer to the backup in case of an error or issue.