To use the SUBSTITUTE or REPLACE function in Excel, follow these steps:

1. Open a new spreadsheet or open an existing one.
2. Click the cell where the replacement is to take place.
3. Type the formula you want to use, either SUBSTITUTE or REPLACE. You will need to insert the relevant text you want to replace, the text you want to replace it with, and the text of the string within which the replacement needs to take place.
4. Once the formula is in place, click on the Enter key.
5. The replaced text will be visible in the cell.