What do I do with the activation key?

I’ve been using the free version and have just now paid and received the activation key.  What do I do with it. Will I be prompted to enter it sometime in the future or is there a place to enter it now? 

Go to Menu > Help > License

Click on Deactivate. This will remove the Free License.

Click on Activate and paste the paste in the activation key of the Pro License.

Got it. Thanks Gary.