Load the backing track into your computer CD drive. Use your preferred audio program, such as iTunes, to save the song in MP3 format. The particular process varies slightly according to which program you are using, but programs typically have the command under the File menu or as a key or mouse command. For example, if using iTunes, right-click the song, select "Create MP3 Version" and wait for alert to tell you the command is complete. Then drag the file onto your desktop.
Double-click the desktop icon to open your preferred digital-audio workstation, such as Logic, Pro Tools or Cubase. If you don't have a digital-audio workstation, there are free open-source programs that provide some of the basic functionality. For example, Audacity has a "Pitch Change" tool that is more than adequate for the purposes of changing the key of a backing track.
Open the "Effects" menu and select "Pitch Change." The effects menu is typically located on the top left of the screen. However, in Apple's Logic program, each individual channel has its own effects menu, located on the channel strip. This opens a new dialog box with key and pitch parameters.
Select the new key. You have the option to enter a specific pitch or alter the pitch by a specific amount, such as semitone. If you don't know which key you want to change the minus track to, adjust the key by semitone increments, hit "Save" and then "Play." Sing along, and if it is still too high or low, select "Undo" from the "Edit" menu and try another key. If you know which key you need, enter it in the box. Digital-audio workstations are capable of altering the key without resampling the audio, meaning the song won't change speed.