The safest method is to log into your Corel account. If you registered your product when you first bought it, the download links should theoretically remain available in your "My Products" section. However, as years pass, companies sometimes retire servers for very old versions. If you cannot find it in your dashboard, contacting Corel Support with your serial number is the recommended first step.
Even though X6 is discontinued, Corel keeps an archive for registered users. coreldraw graphics suite x6 installer
You will need a valid serial number (not included) to activate the software. The safest method is to log into your Corel account