how do i download itunes version 7.5

Depends on what computer you have.

For most, you will need iTunes 10.5.3.

However there is an exception, for the G3 Mac, the latest being 8.2.1 can be downloaded here: http://support.apple.com/kb/DL857

Also, http://www.oldapps.com/itunes.php?old_itunes=28 for previous versions.

you can't. The current iTunes is 10.5.3