How do I delete my payment area from PayPal Standard from a form so that I may add PayPal Pro option?

You can do it by clicking in the red X icon, just click on the payment area and then click on the icon as shown on the image below : 

When you do it, you will receive a message, it asks if you want to keep your products, you should click Yes if you want to keep them and use them with another payment option, in your case with PayPal Pro, then when you add the new payment integration, you products will be there.

Hope this helps.

Regards.