Place your cursor in the cell that holds the existing formula. Then take your cursor to the formula bar and place it next to the equals to symbol (=). Type 'iferror' and press the Tab key to select IFERROR from the list of suggested Excel functions. Move your cursor to the right of the existing formula and enter a comma symbol. Now enter your custom text within double quotes and close the parenthesis of the IFERROR function. Press Enter to generate the results.