To insert a dash between letters and numbers in Excel, use the following steps:

1. Select the cell or range of cells where you want to insert the dashes.
2. Click the Home tab.
3. In the Font group, click the Insert drop-down arrow.
4. Click Special Characters. A dialog box appears.
5. In the Symbol tab, select the character you want to insert. In this case, select the dash (-) character.
6. Click Insert. The character is inserted into the cell or cells.