Basically, a field declared as a FileField, when accessed, gives you an instance of class FieldFile, which gives you several methods to interact with the underlying file. So, what you need to do is:
where new_name is the filename you wish assigned and new_contents is the content of the file. Note that new_contents must be an instance of either django.core.files.File or django.core.files.base.ContentFile (see given links to manual for the details). The two choices boil down to:
# Using File
f = open('/path/to/file')
# Using ContentFile
self.license_file.save(new_name, ContentFile('A string with the file content'))
Hope it helps!!