Unwanted Formular in Excel i need to get rid of Please.

Hello, i had had someone apply the below formular on a spreadsheet i maintain which i am not clear about. apparently this formular is used to append row numbers to the rows in my large excel sheet rather than the manual row number entry. I now don't want this formular and would like to revert back to manual row number entry but don't know what to do. if i delete the formular it also deletes the existing row number and i may loose the ability to match the row numbers to the actual content of the spreadsheet.  Is there a way i can convert my rows back to manual please? 

This formula (="D_4" & TEXT(ROW() - ROW(Table5[[#Headers],[Defect Ref]]) - 400, "0;-0")) generates a custom defect reference code for each row in the “Defect Ref” column of Table5 in the “Defect_Log” sheet.

• It starts with the text "D_4".
• ROW() returns the row number of the current cell.
• ROW(Table5[[#Headers],[Defect Ref]]) returns the row number of the header row for the “Defect Ref” column in Table5 (which is row 3).
• Subtracts 400 from the difference between the current row and the header row, so for row 4: 4 - 3 - 400 = -399.
• TEXT(..., "0;-0") formats the result as a number, showing negative values with a minus sign.
• The formula concatenates "D_4" with the