1. How do I break 16th note. 2. How do I add grace notes

1. Break, into what, 32nds? Just change the duration and add the next note in the place whe there's not a rest, or, in note entry mode, place a 32nd on top of the existing 16th and it'd split into 2 32nds
2. See https://musescore.org/en/handbook/3/grace-notes

1. If you mean breaking the beam connecting 16th notes, see:
   https://musescore.org/en/handbook/3/beams#change-note-beaming
   especially the 'Beam start' symbol:
   https://musescore.org/en/handbook/3/beams#beam-icons

If you mean breaking a 16th note into a 32nd, select the 16th note and press 2 to make a 32nd note.

2. To add a grace note, select a note and double click an icon in the 'Grace Notes' palette. Change the grace note's pitch using up/down arrow.
   See:
   https://musescore.org/en/handbook/3/grace-notes#grace_note