Like this: Tune the G string down a semitone to F#, the C string up a semitone to C#, the E string up two semitones to F#, and the A string up a semitone to A#. Then you would have an F# chord on the open strings.
Oh wait. You didn't really mean to tune the uke did you?
If you just want to play an F# chord, use the F chord shape barred at the first fret -3121