// loop over the add-ins and if you find it uninstall it.
                foreach (AddIn addIn in excel.AddIns)
                    if (addIn.Name.Contains("My Addin"))
                        addin.Installed = false;

                // install the addin
                var addin = excel.AddIns.Add("my_addin.xll", false);
                addin.Installed = true;

var excel = new Application();
var workbook = excel.workbooks.Add(Type.Missing);
excel.RegisterXLL(pathToXll);
excel.ShowExcel();