program.cs

using DocumentFormat.OpenXml.Packaging;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using x = DocumentFormat.OpenXml.Spreadsheet;

class Program
{
    private static readonly string placeHolder = "NamePlaceholder";

    static void Main()
    {
        var templatePath = @"C:\Users\PradeepMurugan\Desktop\Invoices\InvoiceTemplateGst.xlsx";
        var resultPath = @"C:\Users\PradeepMurugan\Desktop\Invoices\test.xlsx";
        string replacementText = "test";
        Dictionary<string, string> keyValuePairs = new Dictionary<string, string>()
        {
            {"NamePlaceholder","Velavan Electricals" },
            {"AddressPlaceholder", "Karur" },
            {"GstinPlaceholder","33251455654" },
            {"InvoiceNumberPlaceholder", "1001" },
            {"DatePlaceholder","16-01-2019" }
        };

        using (Stream xlsxStream = new MemoryStream())
        {
            // Read template from disk
            using (var fileStream = File.OpenRead(templatePath))
                fileStream.CopyTo(xlsxStream);

            // Do replacements
            ProcessTemplate(xlsxStream, keyValuePairs);

            // Reset stream to beginning
            xlsxStream.Seek(0L, SeekOrigin.Begin);

            // Write results back to disk
            using (var resultFile = File.Create(resultPath))
                xlsxStream.CopyTo(resultFile);
        }
    }

    private static void ProcessTemplate(Stream template, Dictionary<string,string> keyValuePairs)
    {
        using (var workbook = SpreadsheetDocument.Open(template, true))
        {
            // Replace shared strings
            SharedStringTablePart sharedStringsPart = workbook.WorkbookPart.SharedStringTablePart;
            IEnumerable<x.Text> sharedStringTextElements = sharedStringsPart.SharedStringTable.Descendants<x.Text>();
            DoReplace(sharedStringTextElements, keyValuePairs);

            // Replace inline strings
            IEnumerable<WorksheetPart> worksheetParts = workbook.GetPartsOfType<WorksheetPart>();
            foreach (var worksheet in worksheetParts)
            {
                var allTextElements = worksheet.Worksheet.Descendants<x.Text>();
                DoReplace(allTextElements, keyValuePairs);
            }

        } // AutoSave enabled
    }

    private static void DoReplace(IEnumerable<x.Text> textElements, Dictionary<string, string> keyValuePairs)
    {
        //textElements.FirstOrDefault(car => car.Text == "Blue");
        foreach (var key in keyValuePairs)
        {
            var item = textElements.Where(x => x.Text == key.Key).FirstOrDefault();
            if (item != null)
            {
                textElements.Select(s => s.Text.Equals(key.Key) ? key.Value : s.Text);
            }
                //text.Text = text.Text.Replace(placeHolder, replacementText);
        }
    }
}