Use Sftp.SetAttributes method:
C#:
// create an Sftp instance and connect/authenticate
Sftp sftp = new Sftp();
sftp.Connect(serveName);
sftp.Login(userName, password);
// specify attributes
SftpAttributes attributes = new SftpAttributes();
attributes.Modified = DateTime.Now;
attributes.Permissions = SftpPermissions.OwnerWrite | SftpPermissions.OwnerRead;
...
// set attributes of a remote file
sftp.SetAttributes("file.txt", attributes);
If you prefer VB.NET, please let me know.