How to set Hyperlink Base programmatically in OpenXML C# in Excel?

Try this once

using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.ExtendedProperties;
using DocumentFormat.OpenXml.Packaging;

public static void SetHyperlinkBase(string filePath, string hyperlinkBase)
{
    using (var document = SpreadsheetDocument.Open(filePath, true))
    {
        var extendedFilePropertiesPart = document.ExtendedFilePropertiesPart;
        if (extendedFilePropertiesPart == null)
        {
            extendedFilePropertiesPart = document.AddNewPart("rId3");
            extendedFilePropertiesPart.Properties = new Properties();
        }

        var documentProperties = extendedFilePropertiesPart.Properties;
        if (documentProperties.HyperlinkBase == null)
        {
            documentProperties.HyperlinkBase = new HyperlinkBase();
        }

        documentProperties.HyperlinkBase.Text = hyperlinkBase;
        extendedFilePropertiesPart.Properties.Save();
    }
}

// how to use
string filePath = "path_to_your_excel_file.xlsx";
string hyperlinkBase = "x";

SetHyperlinkBase(filePath, hyperlinkBase);