Try:
string cmdText = "UPDATE admin SET password ='"+conformpasstxt.Text+"' WHERE username='admin'";
OleDbCommand InsertCommand = new OleDbCommand(cmdText , myOleDBConnection);
Then make sure that the column 'password' in your table admin is of correct datatype.
If this doesn't work, then you need to first check if the update statement is correctly running in your SQL or not.
Try this there:
UPDATE admin SET password ='myPWD' WHERE username='admin'