Add this repository through the Umbrel user interface as shown in the following demo:
sudo ~/umbrel/scripts/repo add https://github.com/decred/umbrel-app-store/
sudo ~/umbrel/scripts/repo updatesudo ~/umbrel/scripts/app install decred-dcrdexsudo ~/umbrel/scripts/repo remove https://github.com/decred/umbrel-app-store/