How do I protect a cell formula from being overwritten

I want to prevent input to a cell to prevent someone from replacing the
formula result with a contsant value.

Hello Wc,

You need to protect your worksheet. On the menu bar go to Tools | Protection | Protect Sheet... A dialog will appear giving you protection options. One of which is using a password to protect the sheet. You can use a password if you want added protection for the worksheet.

If there are cells that will be used for inputting data, you will need to "Unlock" the cells. By default all cells on a worksheet are locked. This is only in effect when the worksheet is protected. On the menu bar go to Format | Cells... | Protection (This is a Tab in the dialog) and click the "Locked" checkbox to clear it.

Sincerely,
Leith Ross