If you are using Datagridview to insert , display and update rows means,
Get the datagridview updated row number and Add 1 . Update the particular row in Excel
int Datagridviewupdatedrow = DataGridViewRowUpdated + 1
WorkSheetApp.Cells[Datagridviewupdatedrow, 1] = cell_value_1;
WorkSheetApp.Cells[Datagridviewupdatedrow, 2] = cell_value_2;
WorkSheetApp.Cells[Datagridviewupdatedrow, 3] = cell_value_3;
If you give a part of code , then will be helpful to explain more.
Thanks