In version 4.2, we’ll be introducing a command to unlink a grand staff automatically, so that you’ll be able to remove the extra staff from the Instruments Menu. But for now, you can still unlink the staff manually by following the steps below. If you need any clarification, don't hesitate to ask.
1. Switch to Select mode by activating the cursor icon from the left side of the Notes Toolbar
2. Open the Copy/Paste menu on the top measure of the grand staff, then drag the bottom-right corner of the blue rectangle so that it covers both staves, as illustrated below:
3. Within the Copy/Paste menu items, first select 'Edit Groups' (as illustrated above), then select "Remove Part Group'. This splits the grand staff into two separate instruments.
You'll then be able to remove the unneeded staff by the following:
4. From the Navigation Menu at the bottom-right corner of the screen, open the Instruments menu popup (icon highlighted below) :
5. Note that the former grand staff is now listed as two separate instruments for the top and bottom staff. Swipe left on the instrument tab corresponding to the unneeded staff, then tap the delete icon that will show up.