I’m not sure what you mean. You can make a Custom Copy of the Fixlet and then edit any file download URL to reference a different file, is that what you mean?
You can upload a new file (usually through a wizard like Manage Software Distribution) and then change the FixletID to reference the new file.
We don’t provide an editor to modify uploaded files (and in most cases we’d expect those uploads to be binaries anyway). If you modify a file referenced by the Fixlet, be advised you also need to modify the Fixlet to reference the new size, sha1, and sha256 hashes.