In version 2.3.5 of PolyNome you can rename a Preset as follows:
1. Select the Preset in the Preset Library.
2. Tap the Actions button and choose "Edit Features"
3. Tap the "Rename Preset" button and choose a new name that isn't already in use.
In previous versions of PolyNome it isn't possible to rename a Preset.
A work around is to save a copy with the new name and delete the original.
1. On the metronome screen tap the disk icon (top right)
2. Choose "Save As"
3. Enter the new name.
4. Delete the old Preset from the Preset Library.
NOTE: Deleting the old Preset will delete any associated Practice events. Any copies of the old Preset that were featured in Playlists will remain (along with their Practice Events)