Without knowing anything about teh error message you get, it is very difficult to be sure, but...
Probably, you problem is to do with the database definition - you have used an Access reserved word
] as a column name, and this is likely to confuse Access when you try to update as it isn't expecting to see the word in that context.
In this case, you should not name any column "name" - if you do, then you need to enclose it in square brackets:
string stdupdt = "update std set [name]='"+txtNM.Text+"' ,addr= '"+txtAD.Text+"' where id='"+txtID.Text+"'";