My music proceeds with a key change from G to C but instead of the expected one natural six naturals are shown. How can I change this?
The indication is correct.
The change is actually from E major (or C♯ minor) to B♠major.
In the E major part the playing instrument is obviously Clarinet in A, but at the moment of the change to B♠Clarinet E major becomes F♯ major, these are the 6 accidentals..
I haven’t tested it but this setting in Layout Options > Players > Instrument Changes might solve the confusion
However you will get one â™® and 5â™ if the key change happens while being in A 
I’d be more worried about the overlapping multi-bar rest. What’s going on there?
You are missing ties in mm.57 and 58.
I have had the same issue with multimeasure rests occurring at an instrument change, but it is easily corrected in the properties panel.
The original transcript I am trying to input into Dorico:
My result in Dorico:
I am confused now. In galley view I can confirm my Bb notes starting at the last measure shown her. To correct the key signature from 6 sharps I made a change at the beginning of that bar and all my music is now correct, however, esthetically I would have liked to get rid of the 6 sharps appearing at rehearsal mark 27 - they do not allow editing! I tried the adjustment you suggested in my layout options but it seemed to make matters worse. I cannot understand why the 6 sharps appear where they do since my instrument change occurs later on. Could it be the cue which is upsetting things? In the test file I have not activate the cue. Finally I enclose a test file.
Test.dorico (1.3 MB)
If you want to reproduce the manuscrit, you could work around this way:
Enter a F major local Key Signature bar 3 in the Bb clarinet staff (will appear as G, i.e. one sharp.) Then, enter a global Bb major next bar.
Page view (I added the bassoon cue):
Test Key Signature Change.dorico (1,4 Mo)
I admit that in this particular case (modulation and transposition change at the same spot) it should be possible to display the cancellation of the last visible Key Signature, i.e. the one of the previous instrument.
Usually you indicate time signature changes at the end of the last measure following a multi rest bar. Why is it putting it at the bar?
Perhaps OT, but I’m confused.
The original score has the first half in E major (concert) and changes to C major (concert) at the 2/4. Surely changing from Cl(A) to Cl(Bb) would mean a key signature of 2# should eventually appear? But the neither the score, nor parts do so… (and yes, the rest of the score is definitely transposed)
Maybe there’s something I’m missing, since you didn’t provide a screenshot of the score. How is it supposed to look like?
Is it a question of Transposed/Concert pitch?
As I suggested earlier, I suspect there is an error in the original score (and part) that you are trying to reproduce.
I don’t understand why the Bb Clarinet does not have a D (2#) key signature at this point in the score since the rest of the orchestra is playing in C major!
(Hopefully the experts will be able chip in)
The original clarinet part is shown at the top of this thread. I have been working in transposed pitch all the time to match my part.
@Janus is right, there is apparently a mistake in the score, isn’t it?
In your first post you speak about a Key change to C, but I thought you were speaking in Transposed pitch terms, since you actually entered Bb (and not C) as a Key Signature at bar 59 (which indeed displays as C in a Bb clarinet part.)
Do you confirm that the general new key signature at bar 59 is C major concert pitch? (I didn’t have a look at the original score as Janus did, but it looks like he’s right!)
If the key signature is C concert pitch, then you’ll need a clarinet in C to display it as C. A Bb clarinet will have a part in D (two sharps)
Or if you want to reproduce what looks like an error, you could enter a local “atonal” KS for the Bb clarinet (or a local Bb major).
I’m a bit late here, but I like this suggestion so much that I would like it to be considered (yet another…) feature request. I don’t have any classic scores handy, but I imagine that this is exactly how these things were combined back in the day, as in the above manuscript example.
The way I’m interpreting it is to Snap instrument transition position to key signature change. If there are multiple key signature changes in between the two instruments, one could plausibly snap to either the first or last intermediate modulation.
In the meantime, of course, we still have your workaround.