Yamaha RY8 Owner's Manual page 37

To change the backing accompaniment, press [2] or [1] to position the
flashing cursor over the name of the backing accompaniment. The backing
name will begin flashing, indicating that you can select a different
accompaniment backing to play with the currently selected rhythm. Then
press [-1] and [+1] to select a backing accompaniment.
The pattern and backing changes you make in the Preset song will only remain active as long as the
current Preset song is selected, or until you select a different measure by pressing [TOP], [BWD] or
[FWD]. When you select another song, the Preset default patterns will be reinstated.
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