Select a cell contains a comment, select Review - Delete (in Comments Group).
Or
Right-click and select Delete Comment from the shortcut menu.
 To delete all Comments in a sheet:
1. Press F5 to open the Go To dialog box, and click Special.
2. In the Go To Special dialog box, select Comments.
3. Click OK. All cells containing Comments are selected.
Or
Skip steps 1 through 3 and press Ctrl+Shift+o (the letter o, not the number zero) to select all Comments.
4. Press Shift+F10 or right-click, and select Delete Comment from the shortcut menu.