There are at least two ways of doing so:
- Static: in other words, it’s programmed into the MIDI (referred to as time-stretching) beat or song. Phil has posted his tutorials on the process using Logic Pro X Tutorial for creating a tempo change in Beat Buddy using Logic Pro X, part 1
- Dynamic: (or real-time) which I think may be what you’re asking for and it’s controlled by a device such as the Arduino. It can also be done with the BeatBuddy (BB). Here are some links.
- https://forum.singularsound.com/search?q=tempo%20changes
- MIDI / BB / VL3 / Arduino Remote - (programming help)
- http://forum.arlomedia.com/index.php?topic=1794.0
- there is or was a link on the forum posted by a user that shared his plans for building a device for controlling tempo on the BB; I can’t find it but you can Google https://duckduckgo.com/?q=arduino+for+beatbuddy&t=osx&ia=videos to get an idea
- you can always buy a MIDI controller