You are right, we did not understand your problem. There is not yet any program that can modify files that are already in the internal memory of the Sony Reader. Files that are on an add-in memory card can be modified when that card is in a card reader attached to your computer.
All of the answers above about modifing the properties meta file data were valid for RTF, LRF, or PDF files. But you are using TXT files which have no Sony Reader defined meta data. The title displayed on the Sony Reader is the file name and the author field is filled in with the date the file was last saved or loaded. As long as you keep using TXT files there is no way to change the title displayed other than to delete the current files from the Reader, rename them on your computer, and then reload them back to the Reader.
That said, many of us have found that the RTF format provides a superior reading experience than TXT files. In addition to being able to select serif or san-serif typefaces, the RTF format allows for meta data so that the books may be accessed by either title or author. Text often times has embedded returns that break lines in odd spots that only worse as the size of the fint increases. Additionally, RTF formatted books may be set in full justification and the interline spacing may be set to your taste. Stingo has developed a macro that runs in MS Word that does a great job of stripping the end of line carrage returns from a TXT file and preparing it for reflowing in the Reader. However the RTF files -- like TXT files -- do not support any graphics or photographs.
Many people have converted their files to LRS which is an unprotected (non-DRM) form of the Sony BBeB format. For example I use Book Designer to prepare files for my Reader and it does a wonderful job.
These free utilities and more are available on the
MobileRead Wiki Conversion page.
Good luck and tell us how it works out for you.