FL4 studio edition, acid, adobe audition, etc. etc. but it takes practice to actually write songs so don't expect something good in the first few tries
The Lastest FL Studio version is 6.0.8, I use it to make my novel's Theme Song
If you want this, Scarch Google with keword "FL Studio". I think You can found it...
I don't think there's software that can actually edit an existing song. When I make my remixes I choose loops in FL Studio that fit what I'm trying to do and rewrite parts of the song while creating other new aspects.