To add a bit more explanations. You basically have three options. Since your table name isn't a reserved word, there's no need to escape it. Because of this the following should suffice:
string sql2 = "UPDATE btechstudent_academics set Tenth=@Tenth Where Regd_no=@Regd_no;";
If it would be a reserved word you would need to escape it. For this you have two techniques. Backticking would always work and the query would look like
string sql2 = "UPDATE `btechstudent_academics` set Tenth=@Tenth Where Regd_no=@Regd_no;";
On the other hand it you have set
ANSI_QUOTES[
^] on you can use quotation marks like this
string sql2 = "UPDATE \"btechstudent_academics\" set Tenth=@Tenth Where Regd_no=@Regd_no;";
or
string sql2 = @"UPDATE "btechstudent_academics" set Tenth=@Tenth Where Regd_no=@Regd_no;";
For more info, please refer to documentation
Schema Object Names[
^]