If you're hunting around the Excel 2007 ribbon for a Change Case button, you may want to try your luck on the Word 2007 ribbon instead. Change Case, the button command in Word that easily converts text to lower case, upper case or fixes toggle case is not an option in Excel 2007. Short of retyping your lower case text with the CAPS LOCK key turned on, there is a function within Excel that you can use to create a formula that will fix the problem.
Insert a blank column to the left of the column with cells that need to be converted to upper case text by right-clicking on the column-header of the source cells and choosing "Insert" from the shortcut menu that appears.
Use the UPPER function to convert the text by clicking in the first cell of the new column and typing the following formula: =UPPER(A1) where A1 is the cell reference of the first cell with lower case text. Use your keyboard to hit the Shift+Enter keys. You will see the result of the formula while staying in the active cell.
Fill the formula down to all additional cells in the new column as needed to convert lower case text from the source column. To fill down easily, position the mouse over the bottom right-hand corner of the current cell, and when the solid, black cross-pointer appears, click and drag the mouse downward. You should now have a new column of upper case text.
Delete the old column of lower case text by right-clicking on the column header and choosing "Delete" from the shortcut menu that appears. If this is not convenient because of your spreadsheet's layout, then copy the cell containing the first UPPER formula and then right-click on top of the first lower case cell, choose "Paste Special" from the shortcut menu and then choose the "Values" option and click the "OK" button.
Fill down the formula to the rest of the lower case cells as needed. This option will overwrite any cells in the range which happen to contain a formula.