To restrict a cell to accepting only "Y" or "N", you can use data validation in Excel. Here are the steps:

1. Select the cell or range of cells that you want to restrict to "Y" or "N".
2. Go to the Data tab in the ribbon and click on Data Validation.
3. In the Data Validation dialog box, under the Settings tab, choose "List" from the Allow drop-down menu.
4. In the Source field, type
   Code:

   "Y,N"

   (without the quotes) or select the cells that contain "Y" and "N" values.
5. Optionally, you can add an Input Message and/or an Error Alert to inform the user about the restriction and what to do if they enter an invalid value.
6. Click OK to apply the data validation.

Now, the selected cell or range of cells will only accept "Y" or "N" values. If the user tries to enter any other value, they will see an error message (if you added an Error Alert) and the value will not be accepted.