Conversion problem sting to int

And the error message is pretty explicit: "Cannot implicitly convert type 'double' to 'string'"

Convert.ToDouble converts a string to a double, so you line is effectively:

double d1 = Convert.ToDouble(txttot1000.Text);
double d2 = Convert.ToDouble(txtcnbamt1000.Text);
txtowndmgamt1000.Text = Math.Round(d1 - d2);

Math.Round takes a double, and returns a double. So your code becomes:

double d1 = Convert.ToDouble(txttot1000.Text);
double d2 = Convert.ToDouble(txtcnbamt1000.Text);
double d3 = Math.Round(d1 - d2);
txtowndmgamt1000.Text = d3;

And the Text property is a string. So what you are trying to do is:

string s = 123.4;

Which is not going to work. Convert it to a string:

string s = 123.4.ToString();

And it'll work.
So:

txtowndmgamt1000.Text = Math.Round(Convert.ToDouble(txttot1000.Text) - Convert.ToDouble(txtcnbamt1000.Text)).ToString();

Is what you need.

But I wouldn't do it that way: if the user enters "Hello" instead ot "123.4" in one of your textboxes, your program will crash. Use Double.TryParse[^] instead, and report problems to the user.