What happens if data is copied in cells that have existing data?
You will initially get a warning asking if you want to overwrite
the existing data. If you say yes, then it will be overwritten.
What effects that will have will depend on what was in those cells.
Sometimes you will want to change the data, so there will be no
real problem. If you overwrite important data that should not be
changed, or overwrite formulas with data, it will affect how your
spreadsheet works. So always be sure you know what you are doing
when overwriting data. Remember too that you can undo if you have
made a mistake. Pressing and holding the Ctrl key and pressing Z
will undo, or you can do it from the Edit menu or toolbar.