Changing key signature of one instrument out of an ensemble

Hi, using MuseScore 3.6. I am trying to change the key signature of the violin in a violin-piano duet. The instruction says to hold down the Ctrl key and guide the selected key signature over the old key signature / into the measure where one wants to change the key signature. When I do as written, nothing happens, the new KS doesn't travel. My Ctrl button functions flawlessly usually. When I hold down the left button of the mouse, whether with or without the Ctrl key, it does travel, but exhibits the red "prohibited" symbol, and doesn't settle. If I select the KS I wish to change, all key signatures change, even though the others weren't selected. My manoeuvres and the red sign aren't showing on the screenshot, unfortunately. I have tried any combination of clicks and movements, with and without the Ctrl, same result. What to do? I need to change it from Abmaj to Amaj. A working solution would be much appreciated :)