Replace text in textbox in Excel using openXML

I want to edit/change the text in an excel sheet which is in a textbox. I iterate through all entries in the sharedstringtable but I cannot find it.

var sharedStringTablePart = spreadSheet.WorkbookPart.GetPartsOfType<SharedStringTablePart>().First();
foreach (SharedStringItem ss in sharedStringTablePart.SharedStringTable.Elements<SharedStringItem>())
{
    if (ss.InnerText == text)
    {
        //Do somethinhg
    }
}

Can someone help?