...is something you can not do at the moment unfortunately! But we hope we can implement this feature in the future, so stay tuned!
In the meantime, there is a workaround if you know the tempo (the BPM) of the track you want to slow down or speed up! Here's an example of how to do it: Say you have a file in 140 BPM that you want to slow down to match a project that is in 120 BPM. Start a new blank project and set the tempo to 140 BPM. Import your audio file and change the tempo to 120 BPM. A popup appears, asking if you want to adjust your audio data to match the new tempo. Click "Adjust". Next, export the individual audio track. Exit your project and return to your profile page. Open the other project in 120 BPM that you were working on. Import the audio file into your project. Depending on how close the starting point of your audio file was to the first downbeat of the music in it, you might have to to some fine editing in order to make sure it lines up and plays completely in sync. You might want to turn off snap to grid mode and zoom in real close when making the edits! Good luck! :)