The apostrophe has been added either by someone or when the data was
exported from another application and in Excel, it means the cell contents
is text. This will also include when a number is in the cell.
1. To remove it, in a blank cell type the number 0.
2. Now copy the cell with 0 in it.
3. Highlight the cells containing the apostrophe and Right Click on the
highlighted area
4. Now Paste Special.
5. Under the Operation part of the Paste Special Menu, select the "Add"
radio button and click OK.
6. The apostrophe has now gone.
Be warned, you may get some unexpected results when you do this especially
with numbers, so try it first on a copy.
Hope this helps.