Change Instrument in Bass Staff

The grand staff is intended for use with a single instrument - e.g. piano or harp. If you want separate staves for two different instruments you should create a score with those instruments - see https://musescore.org/en/handbook/4/setting-your-score#instruments

If you already have the notes in a grand staff you can add another instrument as described here https://musescore.org/en/handbook/4/working-instruments#add-instruments and copy/paste from the grand staff to your new instrument. You can then, if you want, delete the lower stave of the grand staff as described here https://musescore.org/en/handbook/4/working-instruments#delete-staves