What software can change guitar backing track one-half step lower? Many guitar songs are in E and A flat key and backing tracks are in pure E and A respectively, you know. It sounds obviously differently, especially when there is a vocal on record.
Audacity is free, and can do this.
Import the audio file. Highlight it. click Effects > Pitch shift. Select the amount (in semitones)
There are many products out there that can do this, ranging from the really simple "slow-downer" type applications that you can get free for every platform, up to professional products such as Melodyne.
As we aren't a software recommendation site, I'll leave it at that - a google search for pitch-shift will give you an almost endless list.
Transcribe! is such software. Works on Win/Mac/Linux. About $30 and has 1 month free trial. In addition to be able to change the pitch by half-step it can also change it by a percentage (which is useful for some old recordings that are not at A/440). From the "slow-downers" - VLC player, which is also cross-platform and free, can do that (you have to configure it to actually change the pitch when changing speed).
If you want go half step lower you can do it with the help of windows mixer. Firstly you need to right click the speaker icon in your task bar. Then click on 'open volume mixer'. Click on device speaker then. Go to 'Enhancements' tab, Select 'Pitch Shift' Select -1 in the setting drop down menu.
If the track has vocals, it may sound a little funny, but it does the job.