There is a whole chapter in the Mac documentation about how to get information from or write it to filenames, see here:
If you have a specifc problem, then give a real example.
A general hint: if you want to modify a filename, then use the tag information and write a completely new filename. Use Convert>Tag-Filename
If you want to get information from a filename, use Convert>Filename-Tag. If the filename has parts that you don't need, mask these parts with %dummy%