As with nearly all of its parameters, Ableton lets you change the master tempo using an envelope in an automation lane. Unlike most other parameters, though, the master BPM envelope isn’t immediately apparent.
To change the master tempo envelope, switch to Arrangement view by pressing “Tab”, then right-click the BPM display in the upper-left corner of the Ableton window. Select “Show Automation” from the menu that pops up:
Ableton will display a new automation lane in the master channel. Use the two boxes underneath “Song Tempo” to specify the minimum and maximum BPM settings, then draw an envelope to control the tempo. Here, I’ve set the minimum BPM to 100 and the max to 130, then drawn an envelope that takes the tempo from 110 up to 130, then back down to 100:
Use caution with this technique — it can really annoy DJs when a song changes its tempo mid-track.