When you build lighting, the Swarm Agent icon will appear in your Windows taskbar; click it to open the Swarm Agent. Inside, click on “Cache” in the menu. More details are provided here:
https://wiki.unrealengine.com/index.php?title=Swarm_Agent_Troubleshooting#How_do_I_.22Clean.22_and_.22Validate.22_my_Swarm_Agent