I personally use Audio Snap for this with the Radius Mix Offline algorithm. It's really easy to Cntrl A to select all tracks, Open Audio Snap View from the Views tab, Select Clip Follows Project (stretch), then simply Type in your new tempo and preview then offline bounce. You don't have to realign the tempo map with this method.Before doing this I typically bounce all clips to start at zero. It's not necessary, but works fool proof if you do.
If your project contains multiple tempo changes and you want preserve all the ups and downs. Cntrl A to select all tracks, Open Audio Snap View, Select clip follows project, then from the Process tab choose Fit to time, Modify by changing Tempo map (I don't choose Stretch audio there cause I want to preview first then bounce down). For faster tempos choose an earlier Thru time, for slower choose a longer thru time.
To change the tempo of a section, Highlight a section in the Tempo view, hit delete and then insert a new tempo of your choice at the beginning bar. The whole section you deleted will now be what you want your new tempo to be.