First off, please research SQL injections. Your code is extremely vulnerable. You need to pass what is in the textbox as a parameter instead of adding it in the string.
What you are missing are single quotes around the string value. So, what would work is Update studtable set studentmark=studentmark+80 where studentname='bill'. However, do something like this.
SqlCommand cmd = new SqlCommand();
cmd.Connection = sqlConn; cmd.CommandType = CommandType.Text
cmd.CommandText = "Update studtable set studentmark=studentmark+80 where studentname=@studentName"