There is, but it's a bit of a pain, if you're doing the
whole sheet...
On the first column after all your data (i.e. furthest
right hand side of your spreadsheet), write =Lower(first
cell of data) (for example, =Lower(A13)...). You can then
copy and paste this all the way down and over, for as many
rows/columns you require.
Copy the new area, and right click and select Paste
Special and then Values. This will remove the formulae
but leave the new version of the text. You can then
delete the old data.
Alternatively, you can use the same formula on a new
sheet, by typing =Lower( and then clicking the sheet with
the data and pressing enter. Copy and Paste special
(Values) and then delete the old sheet.