To go live on Shopee with a pre-existing video, you can follow these steps: First, log in to your seller account on the Shopee app and access the Shopee Live section. Next, select the live video streaming option and select “Upload pre-existing video” to upload the video you want to use for the live stream. Once the video is uploaded, you can set up relevant information such as title, description, and select products to link to the video. Finally, review the information and click “Start Broadcast” to have your video go live on Shopee.