To change the date format from dd/mm/yyyy to mm/yyyy and get rid of the dd:

1. Select the column with the dates you want to change.
2. Right-click on the selected cells and choose "Format Cells" from the context menu.
3. In the "Format Cells" dialog box, select the "Custom" category.
4. In the "Type" field, enter "mm/yyyy" (without the quotes).
5. Click "OK" to apply the new format to the selected cells.

To get rid of the original value in the formula bar:

1. Assuming your dates are in column A, insert a new column next to it (column B).
2. In cell B1, enter the formula "
   Formula:

   =TEXT(A1,"mm/yyyy") 

   " (without the quotes).
3. Copy the formula down to the rest of the cells in column B.
4. Select the entire column B and copy it.
5. Right-click on the first cell of column B and choose "Paste Special" from the context menu.
6. In the "Paste Special" dialog box, choose "Values" and click "OK".
7. Now you can delete column A, as you no longer need it.

This should give you a new column with only the month and year values, without the original dd/mm/yyyy value visible in the formula bar. You can use this new column for your pivot table analysis.