You need to revert a data value change for a field on multiple leads.
There is no way through the UI to "undo" a data value change (unless it was a boolean field, in which case there are only two possible values.) You can certainly use a Smart List to identify whose values have changed, but there no way to go back to the old value (unless it's stored elsewhere in another field or through another data store, along with the API).
However, you can use the API to extract the data value change activity from the activity log, which would include the old and new values, extract the original values, then use the API to rewrite the old values back to the lead record. http://developers.marketo.com/rest-api/endpoint-reference/lead-database-endpoint-reference/#!/Activi...
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