Assuming that column A, cells 1 through 5 contain the text you wish to convert to all uppercase:
Insert a column to the right of column A by selecting column B and clicking Columns on the Insert menu. (You don't need to do this if column B is empty.)
Type the following formula in cell B1 =UPPER(A1)
In column B, select all the cells to the right of the cells that contain data in column A. In this example it would be cells B1 through B5.
On the Edit menu, point to Fill, and then click Down. Column B now contains the uppercase version of the text in column A.
With the same range selected, click Copy on the Edit menu.
Select cell A1, and click Paste Special on the Edit menu. In the Paste Special window, click Values, and click OK to past the converted values back on top of column A.
Delete the contents of column B. (If this was a column you inserted, delete the entire column by selecting the column and clicking Delete on the Edit menu.
You can do the reverse of changing text to all lower case by substituting the formula with =LOWER(cell range).