The documentation says : https://firebase.google.com/docs/firestore/query-data/indexing
To delete an index:
Go to the Cloud Firestore section of the Firebase console. Click the Indexes tab. Hover over the index you want to delete and select Delete from the context menu. Confirm that you want to delete it by clicking Delete from the alert.
On current Cloud Firestore ,
I have only found a console command to delete all indexed for a firestore db
I have same issue with firestore UI web panel tho I found a solution using cloud.
First you need to enter https://console.cloud.google.com/ and select your project after that open gcloud terminal (you can access this on the top right corner nearby your avatar).
To list all your composite indexes for selected project, type this into the terminal
gcloud firestore indexes composite list
Then pick id
of the one you want to remove and type
gcloud firestore indexes composite delete <composite-id>
It'll ask you for confirmation, once you done it by typing and entering y
your index is gone c: