Apply partial styling to text in excel cell using EPpplus
You should try to use ExcelRichText class. For example:
var newFile = new FileInfo("example.xlsx");
using (var package = new ExcelPackage(newFile))
{
var worksheet = package.Workbook.Worksheets.Add("Example");
var boldRichText = worksheet.Cells[1, 1].RichText.Add("Hello");
boldRichText.Bold = true;
var normalRichText = worksheet.Cells[1, 1].RichText.Add(" World");
normalRichText.Bold = false;
package.Save();
}