rename files

Suppose I have many files which names are like:

990323 sth sth
990568 sth sth
990898 sth sth

suppose there are thousands of files like that. These are in a folder and I would like to rename them, not one by one, but automatically to something like:

995323 sth sth
995568 sth sth
995898 sth sth

I mean, replacing “0” by “5” in the middle of the first part of these names. Remember the files name are longer names of Windows.
How could I proceed to the name changing of these files, without having to make it one by one thousands of time by hand.
I accept any suggestion including using delphi or any other resource of the Windows.