Equivalent keys in transposing instruments.

To change the flats to the appropriate sharps, select the section of music in instrument's line of music you want fixed only. Make sure no other instruments are selected unless you want their key signature changed also. Use the menu Notes->Transpose... select the closest key below it that contains sharps, in the case of the A clarinet use F#Maj/D#min. Make sure you have By Key check, Transpose Down selected and Transpose Key Signature checked. Keep the same line of music selected (it should still be). Now use the Transpose... dialog again, this time check By Interval and keep Transpose Key Signature. You need to now figure out how far to transpose it to return it to the key of concert Ab(G#)Maj. In this case it is a major second, so in the interval change perfect union to Major Second. When you press "OK" you will have 5 sharps instead of 7 flats for the A Clarinet.

The principle works the same for all other transposing instruments that give you undesirable results.

Some final notes, there do not need to be notes in the section of music you fix. The key signature is fixed and will work as with any other key signature. The key signature can only have sharps and flats, MuseScore will not create a time signature with double sharps or double flats.