Change Clarinet from A to B tuned

You don't need to change the key signature. Allways add the correct concert pitch (sounding pitch} key signature. Musescore automatically uses the correct transposed key signature if you view the score at written pitch - without the "concert pitch" button selected.

Start by choosing Bb Clarinet when you create the score. See https://musescore.org/en/handbook/3/create-new-score#instruments-and-vo… . If you later need to change to A Clarinet add an instrument change text from the [Add]>[Text] menu and select Clarinet in A from the orchestral list. See https://musescore.org/en/handbook/3/mid-staff-instrument-changes

Note: Do not add the Instrument change text from the text pallet. That one is broken and does not work with transposing instruments.

The score in your picture is written in D major or B minor.
You have chosen a B-flat clarinet and this instrument will have E major or C-sharp minor in this score.
Simply: you've chosen the wrong clarinet type ...

If you want to have an A clarinet, just right-click on a clarinet staff - staff/voice properties - change instrument. Enter "A clarinet" in the search window and you will have this notation variant in your score, in F major or D minor. Write the notes.
If you have already written the notes of the A clarinet on the staff of the B clarinet, you must move them down two semitones. Select them and press the down arrow key twice.

Or: Add a new instrument (i), select A clarinet and then write the notes for the A clarinet. That should have been the correct way at the score setup.