Create a ZipArchive from the ZipFile, and use the ExtractToFile method if the file type is acceptable:
using (ZipArchive zip = ZipFile.Open(zipfile, ZipArchiveMode.Read))
foreach (ZipArchiveEntry entry in zip.Entries)
{
string ext = Path.GetExtension(entry.Name).ToLower();
if(ext == ".txt" || ext == ".xlsx")
{
... sort out where you want to put it ...
entry.ExtractToFile(outputFileNameAndPath);
}
}