diff --git a/howto/prometheus.md b/howto/prometheus.md
index 4098c33a6b474feb4bbdaa5058f94c60ef1967c0..fb943bd92c38ea3da6c84e5091fe24946c6d72d0 100644
--- a/howto/prometheus.md
+++ b/howto/prometheus.md
@@ -406,6 +406,41 @@ configured, inside Puppet, in `profile::prometheus::server::external`.
 Note that it's [not possible to push timestamps](https://github.com/prometheus/pushgateway#about-timestamps) into the
 Pushgateway, so it's not useful to ingest past historical data.
 
+## Deleting metrics
+
+Deleting metrics can be done through the Admin API. That first needs
+to be enabled in `/etc/default/prometheus`, by adding
+`--web.enable-admin-api` to the `ARGS` list, then Prometheus needs to
+be restarted:
+
+    service prometheus restart
+
+WARNING: make sure there is authentication in front of Prometheus
+because this could expose the server to more destruction.
+
+Then you need to issue a special query through the API. This, for
+example, will wipe all metrics associated with the given instance:
+
+    curl -X POST -g 'http://localhost:9090/api/v1/admin/tsdb/delete_series?match[]={instance="gitlab-02.torproject.org:9101"}'
+
+The same, but only for about an hour, good for testing that only the
+wanted metrics are destroyed:
+
+    curl -X POST -g 'http://localhost:9090/api/v1/admin/tsdb/delete_series?match[]={instance="gitlab-02.torproject.org:9101"}&start=2021-10-25T19:00:00Z&end=2021-10-25T20:00:00Z'
+
+To match only a job on a specific instance:
+
+    curl -X POST -g 'http://localhost:9090/api/v1/admin/tsdb/delete_series?match[]={instance="gitlab-02.torproject.org:9101"}&match[]={job="gitlab"}'
+
+Deleted metrics are not necessarily immediately removed from disk but
+are "eligible for compaction". Changes *should* show up immediately
+however. The "Clean Tombstones" should be used to remove samples from
+disk, if that's absolutely necessary:
+
+    curl -XPOST http://localhost:9090/api/v1/admin/tsdb/clean_tombstones
+
+Make sure to disable the Admin API when done.
+
 ## Pager playbook
 
 TBD.