1.    Firstly, we need to deactivate the current SOLIDWORKS License in your computer. Open SOLIDWORKS, click menu Help->Licenses->Deactivate.




2.    Now Go to Windows Setting->Apps  

3.    In the list we can search to find SOLIDWORKS, then click Modify.   




At the next page we need to fill in the new Serial Number for version 2019-2020 (How to get the new serial code - https://gtiithelpdesk.freshservice.com/support/solutions/articles/15000028853). 



Then you had successfully changed the Serial number of SOLIDWORKS.