Does anyone know how to change the tempo of a project in the middle of a song and then return back to the original tempo?

place the NOW time where you want the tempo change => views / tempo => click the '+' symbol => enter new tempo
 
likewise for later tempo change ...
 
or draw with mouse in tempo view ...

Thanks!

Keep in mind that if you are using VSTi's in your projects, some will react badly to the tempo change with audible artifacts like doppler-sounding delays, reverb glitches, and other unexpected noises. Just making you aware.
 
To avoid these issues, you will need to be creative about how you handle the transitions, or you may want to bounce sections with the tempo changes inactive. You'll know it when it happens.
 
Regards,
Dan