obviously this is an ASP.NET message board if i am not mistaken. But it seem that you are sending the Update Method a string ("me"), if you set your update command as default it should except strings, integers, bool as default values for each item in the table. maybe you should be doing something more like
da.Update(txtFirstName.text, txtLastName.Text)
Anyways when you open the method, opening it would be putting the "(" after the method name, it should display a tool tip with what the method needs sent it. It should look like (string FirstName,string LastName) and you have to match each item with what value you want sent it..... just take a look.
Hope that helps in some way.