Fix C# app can not connect to server because SSL certificate

I already have website for food online ordering which is made by core PHP and there is C# app which is installed in windows 7 computer, this C# app receives any orders which are coming from the website. Everything was working well until yesterday when the C# app could not connect to my server and this is because of windows 7 TLS or SSL is no longer supported or I think because other security issues, I want someone to check and fix it.