How Do I Connect Excel Spreadsheet To A C# Application ?

If just read and update ,that's very simple,you can create your own operation surface,then in the part of realizing function,you can try this excel library[^] .Download a free edition (which can access most of it's supported funtion),and add reference to your project.
Read Excel:

//Create a new workbook
Workbook workbook = new Workbook();       
//Load a file and imports its data
workbook.LoadFromFile(@"..\FandH.xlsx");  

Edit cell:

//Edit Cell Value
          sheet.Range["E2"].Value = "00-1-285-7901742";
          sheet.Range["E2"].Style.Font.FontName = "Book Antiqua";
          sheet.Range["E2"].Style.Font.Color = Color.DarkOrange;

Other functions as conversion or cell merge is included, more tutorials .
here[^]

Step1:Upload your spreadsheet in some temporary folder

private void btnUpload_Click(object sender, EventArgs e)
       {
           string sourceFile = fileDlg.FileName;
           string[] fileName = sourceFile.Split('\\');

           if (!Directory.Exists(TempDirectory))
           {
               Directory.CreateDirectory(TempDirectory);
           }

           string destinationFile = TempDirectory + fileName[fileName.Length - 1];

           if (File.Exists(sourceFile) && !File.Exists(destinationFile))
           {
               File.Copy(sourceFile, destinationFile);
               MessageBox.Show("File copied...", "Notification");
           }
           else if (!File.Exists(destinationFile))
           {
               MessageBox.Show("Select a file...", "Error");
           }
           else
           {
               MessageBox.Show("File already exist...", "Notification");
           }

           txtBoxFile.Text = string.Empty;
           GetAvailableFiles();
       }

private void GetAvailableFiles()
      {
          cbAllFiles.Items.Clear();
          cbSheets.Items.Clear();
          cbAllFiles.Text = "Select";
          cbSheets.Text = "Select";
          string[] allfiles = Directory.GetFiles(TempDirectory);
          var fileList = (from f in allfiles select f.Split('\\')[f.Split('\\').Length - 1]).ToArray<string>();
          var extList = new List<string>() { ".xls", ".xlsx" };
          fileList = fileList.Where(f => extList.Contains(Path.GetExtension(f)) && !f.StartsWith("~")).ToArray<string>();
          cbAllFiles.Items.AddRange(fileList);
      }

N.B:
cbAllfiles is ID of a combobox
cbSheets is ID of another combobox which tracks the sheet name

For getting the sheet name from cbAllFileName selected index changed event:

private void cbAllFiles_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (((ComboBox)sender).SelectedIndex != -1)
            {
                string DB_Path = string.Format("{0}{1}", TempDirectory, cbAllFiles.SelectedItem.ToString());
                string fileExt = Path.GetExtension(DB_Path);

                if (fileExt == ".xlsx")
                    ExcelConnection = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + DB_Path + ";Extended Properties=Excel 12.0;";
                else
                    ExcelConnection = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + DB_Path + ";Extended Properties=\"Excel 8.0;HDR=Yes;IMEX=1\";";

                try
                {
                    OleDbConnection connExcel = new OleDbConnection(ExcelConnection);
                    connExcel.Open();
                    DataTable schemaTable = connExcel.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);

                    String sheetName;
                    List<string> lstsheetNames = new List<string>();

                    foreach (DataRow row in schemaTable.Rows)
                    {
                        sheetName = row.Field<string>("TABLE_NAME").Trim();

                        if (sheetName.Length > 1 && sheetName.EndsWith("$"))
                        {
                            
                                lstsheetNames.Add(sheetName.Substring(0, sheetName.Length - 1));
                        
                       
                    }

                    cbSheets.Items.Clear();
                    cbSheets.Items.AddRange(lstsheetNames.ToArray());
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message + " StackTrace: " + ex.StackTrace, "Exception");
                }
            }
        }

Step-2:

string excelName = Path.GetFileNameWithoutExtension(cbAllFiles.SelectedItem.ToString());

 string sheetName = cbSheets.SelectedItem.ToString();

//This is where you can get all the rows details in a datatable

DataTable dt = DataManager.GetSheetData(sheetName, ExcelConnection);

public static DataTable GetSheetData(string sheetName, string excelConn)
        {
            DBAccess db = new DBAccess(excelConn);
            string cmdText = "Select * from [" + sheetName + "$]";
            db.CommandText = cmdText;
            return db.ExecuteExcelDataSet();
        }

If you feel this is a right solution then tick on green color button please.
Thanks,