By default, harp pedal diagrams are centered vertically between the two staves usually shown for harps.
You can move harp pedal diagrams to different rhythmic positions in Write mode. They are positioned by default according to your settings in Engraving Options.
You can move harp pedal diagrams graphically in Engrave mode, but this does not change the rhythmic positions to which they are attached.
You can change the default positions of all harp pedal diagrams project-wide on the Harp Pedaling page in Engraving Options.