Debi, so far, nobody has asked you why you want to tune the melody string to F#.
One guess I have would be: because you want to play a tune that you were told is in the key of F#? If that's the case, then you would not be able to do that while leaving the other two strings in D and A.
Another guess I have would be: that you are thinking about the tuning D-F#-A, which some people have used in the past as an 'easy' 1-3-5 tuning to play in on the dulcimer, because it allows you to play bar chords a lot. But playing in D-F#-A is not the same as playing in D-A-F#.

If you tell us your goal and your reason for wanting to retune your melody string to F#, we can much better help you.

That's it on the tuning. Usually you can go higher/lower 2 half steps with no problems. A half step is one note up/down on a chromatic scale, ie; F to F#, or B to C. D to E is a full step, D-D#/Eb-E.

Thx, to everyone that responded.  Looks like a have a bit of music theory work in my future.  Just to make sure that I'm clear,  Bass string is D (D3), Middle string is A (A3) and melody string is d (d4).  Also, if I understand correctly, you are better off trying to tune a string down rather than up and risk breaking it.

Thx again, everyone.  

Happy Holidays to all!

I'd also point out that if your dulcimer has a standard VSL (length from nut to bridge) of between 26-28", and you currently have fairly standard gauge strings on it, then if you tune UP from the note of D on EITHER your bass string (low D) or your melody string (high d) two whole steps to F#... then there is a very strong likelihood of you breaking that string with too high a tension.

You'd be better off tuning from d DOWN to the next lower F#. However, that's a lot lower than it currently would be, and many folks might find it feels too slack unless they change to a heavier string.

I agree that Dan and Skip's posts are a good thing to determine first. But i though t i'd mention about possibly tuning too high and breaking a string.

What @skip said!

You almost have it! Instead of high to low, dAD, use low to high, DAd, it is more conventional for MD. For the other part of your question, always pluck the string just before turning the knob on the tuner. That way you know when you tune in the correct direction. As far as the F# A d, that would require a string change. You may be thinking of DF#A which needs different strings also. If you are tuned to DAA, just tune down, loosen, the middle string to F#.

DAG is Dorian, DAF is Phrygian, both will probably need restringing unless you are tuned DAA.

dAF#/DAF# is not a mode.

Hi Dan,  I’m not sure how to answer that.  I believe that I’m in mixolydian when tuned dAD.  I’m trying to retune the melody string to F# and don’t know if I should tune up or down to the F#.  Does that put me into a different mode?

Maybe if we could get the mode you are trying to get to?

Hi Dulcimer Folks,

When you are retuning from to dAD (hoping I wrote this correctly) to dAF# to play a different tune,  if you have not already heard the tune, how do you know which direction to tune the string, up or down in pitch.  Hope this question makes sense.  Thx.