To automate the tempo of a Preset:

1. Select the Preset

2. Tap the Actions button

3. Choose "Add AutoTempoChange"

A Playlist will be created from that Preset and some basic tempo automation applied.

You can then modify the tempo automation by tapping the Playlist Settings row at the bottom of the playlist.

NOTE: The default tempo automation settings are pulled from the default Playlist (it's called "Playlists Introduction" and you'll find it in Playlist Manager). If you load Playlists Introduction and modify settings in the Playlist Controls row, they'll be used next time you create a tempo automation.