To quickly remove the metadata from your instances, select them on the Stage and press Remove Rig info button (X) in SMR Helpers panel. When this is done, you will be able to re-rig them.
If you want to fully remove everything, Remove Rig info from all the Symbol Instances and also delete the EDAPT objects folder (which contains Magnet Target and Center Marker) from the Library.
To reset the matrices of your instances, use Flash/Animate's Transform panel. There is a Remove Transform button at bottom-right.
The information you were looking for is here: https://flash-powertools.com/smart-magnet-rigging-process/#sr05
Please, if you want to learn how to use SMR, read all articles in the series, which start here: https://flash-powertools.com/smart-magnet-rigs/