The DWG have had a number of reports about this vandalism (as you’d expect). In most (at the time of writing, probably all) cases, it has already been reverted**.
However, if you see something on a rendered map such as at https://www.openstreetmap.org you’ll want to check that the problem has actually been fixed. In order to do that, go to https://www.openstreetmap.org and zoom right in to where there was a problem. Use the “?” button at osm.org and click on the map where the problem was. A list of “nearby features” will appear. If one of them looks wrong, please tell the DWG about it. If nothing looks wrong any more, then the data has been fixed and you don’t need to report it.
However, once the data is fixed, various maps and applications based on OSM data will still take time to get updated. If you think that a map or app displays incorrect data, please raise that issue with whoever is responsible for that map or app. If you are unsure how to contact whoever it is, then asking a question in the forum here is a good place to start.
** Edit: Some relations may still have some issues (at first glance this may be due to people trying to do reverts using buggy software), but the scale of the problem should be manageable. I’ll update here later.
Edit: I was only able to find one problem relation that needed fixing, and that’s now fixed. Another DWG member found one other. If there are more, please email data@openstreetmap.org to let the DWG know.
Another edit (9 days later) someone reported one more broken relation to us yesterday.