When the trial period ends for your SOLIDWORKS Product and you´ve decided to continue using SOLIDWORKS products and bought the license you need to change license.
There is no need to unistall existing software since you only want to change the license.
To do that access Add or Remove programs and locate your SOLIDWORKS installation, click on it and select "Modify":
Once "Modify" has been selected "SOLIDWORKS 20XX SP X Installation Manager" window will open, select "Modify your installation" and click "Next":
On the following screen you´ll see SOLIDWORKS Product licenses and you need to locate license beginning with 8800 XXXX XXXX XXXX XXXX XXXX. In this example I´m going to change 3D Design SOLIDWORKS license.
Locate your new SOLIDWORKS product license starting with 9000 XXXX XXXX XXXX XXXX XXXX. Type the new license instead of the trial/ecaluation license (it starts with 8800) and click "Next".
Follow the dialog until modifications have been made. Now you can start using your new SOLIDWORKS license.