You can protect cells from changes
by setting cell protection.
Cell protection is activated for all cells as the default and it is
only effective after you protect the sheet or the document.
You
can click to protect the
sheet or the document. The protection becomes active when you close
the dialog with OK, even if you do not specify a password. If you
enter a password and then later forget it, you will have no way to
deactivate the protection. If you merely want to protect the cells
from inadvertent changes, it is better not to enter a password.