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Notate it enharmonically as Fb. It may look strange to you but a harpist won't be phased by it, they are used to seeing things like this all the time. You need to indicate the change of tuning in the space between the bass and treble clefs but it ideally needs to be done a few beats ahead of time as the 3 pedals that would need changing on this occasion (assuming you were just in C major to begin with, I don't really know the context of your piece): A, E and F, are all on the right side of the harp so they all have to be changed separately with the harpist's right foot and harpists prefer to change pedals once on the beat if possible. If you want to get a better feel for harp notation why don't you type something like "harp concerto" into IMSLP and have a look at a few scores to see some real examples.