First get the textbox value in integer variable and than add 1 to it..
int txtboxText = Convert.ToInt32(txt1.Text);
int result = txtboxText + 1;
OR U can do this in a single line.
int result = Convert.ToInt32(txtbox.text) + 1;
and to restrict user to enter only numeric values to textbox You can use Filtered textbox extender.