I am sorry mate, it seems you are not only new to SqlClient but also totally new to SQL. I would recommend reading on how UPDATE and INSERT work, have a look around for sample code as to how to do UPDATE and INSERT using SqlCommand. This is not a tutorial section rather a "Quick Answer" section. I have already explained a lot. But i will give you some algorithm in plain English as to what you should be doing:
--get the record count from the table
--if (count == 0)
----execute the INSERT command
--else
----execute the UPDATE command
Also, a couple of points
1) parameterise the INSERT query too like you did for the UPDATE query.
2) instead of doing a SELECT * FROM <tablename>, do a SELECT COUNT(*) FROM <tablename>, this will just give you the record count directly rather than loading the whole table. Unless, you need the table data as well.
That's more than anyone will help you here trying to get one point across. Your code is not wrong, it just needs some rearranging as I mentioned above.
Cheers...