You can convert your trial to a paid subscription at any time by clicking "Buy Now" in the trial message window.
Some users attempt to "reset" the trial period to get another 15 days. These methods are unofficial, may violate the End User License Agreement (EULA), and could lead to software instability. how to extend coreldraw trial
If your 15-day trial has ended and you need more time, the following are the only legitimate paths forward: You can convert your trial to a paid