If you had previously chosen to auto-update, open Lightroom Classic and go to Help>System Info to verify that you are now using 10.4. If not, or if you hadn’t chosen to auto-update, go to Help>Update, and the Creative Cloud app will open. Click on Update next to Lightroom Classic. If the update is not listed in the CC app then go to Help>Check for Updates. If it’s still not there, click on the three dots next to Lightroom Classic, choose Other Versions, and see if 10.4 is listed there. Otherwise if it’s the day of release, then try later – it may take 24 hours to propagate worldwide.
This update process is covered in my video tutorial below: